Provide ChannelHandler that closes Channel on unhandled error.

[Lukasa]

As we change #600, many users will start leaking Channels because they don't close on error. We should provide a ChannelHandler that closes on unhandled error that is typically added last to the pipeline.

[weissi]

see also:

• the NIO backends should close the Channel if an error is received in the last ChannelHandler grpc/grpc-swift#381
• https://github.com/pointfreeco/swift-web/issues/182
• MicroExpress should close the Channel if an error is received in the last ChannelHandler NozeIO/MicroExpress#8
• swift-talk-backend should close the Channel if an error is received in the last ChannelHandler objcio/swift-talk-backend#103
• Perfect-NIO should close the Channel if it receives an error in the last ChannelHandler PerfectlySoft/Perfect-NIO#6
• Vapor should close the Channel when it receives an Error in the last ChannelHandler vapor/http#334
• Smoke should close the Channel if it receives an error in the last ChannelHandler amzn/smoke-framework#19
• KituraNIO should close the connection if it receives an error in its last ChannelHandler Kitura/Kitura-NIO#173

[allright]

I think this tickets are very similar:
#841