core,pm: handle zero/nil gas price from gas price monitor

[kyriediculous]

What does this pull request do? Explain your changes. (required)
When the gas price monitor returns a gas price of 0 or nil in on-chain mode the node will crash with a division-by-zero error. This bug caused a majority of the orchestrators on the public network to all crash simultaneously when a common hosted ethereum provider, infura, could have potentially returned a gas price of 0. (see below).

Specific updates (required)

• In recipient.txCost() if the gasPriceMonitor returns 0 (or nil , but this should be impossible due to the above update) txCost will return 0
• In recipient.TxCostMultiplier() if the value returned from recipient.txCost() is 0 , return big.NewRat(0,1)
• In orchestrator.priceInfo() if the txCostMultiplier return from recipient.TxCostMultiplier() is 0/1 use a price multiplier of 1 (no overhead charged because of no tx cost).

How did you test each of these updates (required)

• Added unit tests
• Manual test on private network with 0 gas price

Motivation
Decided to add extra checks to deal with cases of nil and 0 values down the call chain. While this is a more elaborate fix it would prevent breaking the software on private networks with a gas price of 0 or running against an ethereum node that also mines with a gas price of 0.

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

Context

panic: division by zero

goroutine 336441272 [running]:
math/big.(*Rat).SetFrac(0xc003369480, 0xc00019aea0, 0xc000ccfb20, 0x1ee6364f)
        /usr/local/go/src/math/big/rat.go:307 +0x465
github.com/livepeer/go-livepeer/pm.(*recipient).TxCostMultiplier(0xc0001780c0, 0x7d1b06f7c8c4c7c3, 0xfe5953ee6e76726a, 0xc01ee6364f, 0x8361c8, 0xc001bee460, 0xc001bee440)
        /root/go-livepool/pm/recipient.go:289 +0xce
github.com/livepeer/go-livepeer/core.(*orchestrator).priceInfo(0xc000512910, 0x7d1b06f7c8c4c7c3, 0xfe5953ee6e76726a, 0x1ee6364f, 0x0, 0xc001156a20, 0xc0023b58b0)
        /root/go-livepool/core/orchestrator.go:283 +0x22d
github.com/livepeer/go-livepeer/core.(*orchestrator).PriceInfo(0xc000512910, 0x7d1b06f7c8c4c7c3, 0xfe5953ee6e76726a, 0x1ee6364f, 0x0, 0x20, 0xc001bee4c0)
        /root/go-livepool/core/orchestrator.go:261 +0x96
github.com/livepeer/go-livepeer/server.getPriceInfo(0x1fccc58, 0xc000512910, 0x7d1b06f7c8c4c7c3, 0xfe5953ee6e76726a, 0xc01ee6364f, 0xc0023b5988, 0x690ba7, 0x0)
        /root/go-livepool/server/rpc.go:282 +0x53
github.com/livepeer/go-livepeer/server.orchestratorInfo(0x1fccc58, 0xc000512910, 0x7d1b06f7c8c4c7c3, 0xfe5953ee6e76726a, 0x1ee6364f, 0xc001bee4c0, 0x1c, 0x50, 0x0, 0x0)
        /root/go-livepool/server/rpc.go:286 +0x6a
github.com/livepeer/go-livepeer/server.getOrchestrator(0x1fccc58, 0xc000512910, 0xc0042c35e0, 0x1c51840, 0xc004ed9600, 0x1fad5d0)
        /root/go-livepool/server/rpc.go:272 +0x305
github.com/livepeer/go-livepeer/server.(*lphttp).GetOrchestrator(0xc0001418e0, 0x1fbc0f0, 0xc004ed9680, 0xc0042c35e0, 0xc0001418e0, 0xc004ed9680, 0xc000067b88)
        /root/go-livepool/server/rpc.go:140 +0x45
github.com/livepeer/go-livepeer/net._Orchestrator_GetOrchestrator_Handler(0x1c51840, 0xc0001418e0, 0x1fbc0f0, 0xc004ed9680, 0xc0050104e0, 0x0, 0x1fbc0f0, 0xc004ed9680, 0xc0067d9860, 0x59)
        /root/go-livepool/net/lp_rpc.pb.go:1688 +0x214
google.golang.org/grpc.(*Server).processUnaryRPC(0xc00011aea0, 0x1fc7658, 0xc003d01320, 0xc00ffa0a00, 0xc00011cf00, 0x2a60960, 0x0, 0x0, 0x0)
        /root/go/pkg/mod/google.golang.org/grpc@v1.28.0/server.go:1082 +0x52b
google.golang.org/grpc.(*Server).handleStream(0xc00011aea0, 0x1fc7658, 0xc003d01320, 0xc00ffa0a00, 0x0)
        /root/go/pkg/mod/google.golang.org/grpc@v1.28.0/server.go:1405 +0xccf
google.golang.org/grpc.(*Server).serveStreams.func1.1(0xc000500a20, 0xc00011aea0, 0x1fc7658, 0xc003d01320, 0xc00ffa0a00)
        /root/go/pkg/mod/google.golang.org/grpc@v1.28.0/server.go:746 +0xab
created by google.golang.org/grpc.(*Server).serveStreams.func1
        /root/go/pkg/mod/google.golang.org/grpc@v1.28.0/server.go:744 +0xa5
       ```

[kyriediculous]

I rebased already so I could drop the gaspricemonitor commit and the diff is really tiny

[yondonfu]

LGTM