Fix private channel resubscribe deadlock

[FZambia]

Fixes deadlock in situation when client resubscribes and tries to get channel private signature.

#	0x1043386	sync.runtime_SemacquireMutex+0x46						/usr/local/Cellar/go/1.13/libexec/src/runtime/sema.go:71
#	0x2f819b1	sync.(*RWMutex).RLock+0xc1							/usr/local/Cellar/go/1.13/libexec/src/sync/rwmutex.go:50
#	0x2f81919	github.com/centrifugal/centrifuge-go.(*Client).clientID+0x29			/go/pkg/mod/github.com/centrifugal/centrifuge-go@v0.6.1/client.go:189
#	0x2f866ec	github.com/centrifugal/centrifuge-go.(*Client).privateSign+0x7c			/go/pkg/mod/github.com/centrifugal/centrifuge-go@v0.6.1/client.go:1133
#	0x2f8ae7b	github.com/centrifugal/centrifuge-go.(*Subscription).resubscribe+0xdb		/go/pkg/mod/github.com/centrifugal/centrifuge-go@v0.6.1/subscription.go:453
#	0x2f85799	github.com/centrifugal/centrifuge-go.(*Client).resubscribe+0xa9			/go/pkg/mod/github.com/centrifugal/centrifuge-go@v0.6.1/client.go:918
#	0x2f8e1d5	github.com/centrifugal/centrifuge-go.(*Client).connectFromScratch.func1+0x865	/go/pkg/mod/github.com/centrifugal/centrifuge-go@v0.6.1/client.go:895
#	0x2f8eb33	github.com/centrifugal/centrifuge-go.(*Client).sendConnect.func1+0x1c3		/go/pkg/mod/github.com/centrifugal/centrifuge-go@v0.6.1/client.go:1123
#	0x2f835f6	github.com/centrifugal/centrifuge-go.(*Client).handle+0x136			/go/pkg/mod/github.com/centrifugal/centrifuge-go@v0.6.1/client.go:495
#	0x2f83374	github.com/centrifugal/centrifuge-go.(*Client).readOnce+0x94			/go/pkg/mod/github.com/centrifugal/centrifuge-go@v0.6.1/client.go:467
#	0x2f83488	github.com/centrifugal/centrifuge-go.(*Client).reader+0x78			/go/pkg/mod/github.com/centrifugal/centrifuge-go@v0.6.1/client.go:477