Cancellation support.

[cheatfate]

Currently nim-libp2p do not have cancellation support, and this can be one more source of leaks for applications which is going to use it.

All API calls which performs open(dial, connect)/read/write operations should handle cancellation and perform cleanup of allocated resources and/or reset stream operations.

Example of leak can be seen here:
https://github.com/status-im/nim-beacon-chain/blob/master/beacon_chain/eth2_network.nim#L433-L442

Any other procedure which will going to use read/connect/dial with timeout should be able to cancel pending read/connect/dial operation.

[cheatfate]

@dryajov sorry but its not an enhancement, its missing feature which leads to leaks.

[dryajov]

This has been improved a lot but there are still a few flows that need revisiting, for example the upgrade flow is now extracted from the switch and we can test it separately.