Update coroutines

[whyoleg]

• minor: kotlin 1.4.21
• coroutines 1.4.2-native-mt
• Use Flow.stateIn operator from coroutines 1.4.x in reconnectable implementation
• minor: Rename Cancelable to Cancellable (double L)
• use CompletableJob in Cancellable interface for better connection state handling
• use CompletableJob.completeExceptionally instead of Job.cancel in places, where error should happen
• use new channel API with onUndeliveredElement (yet workaround for Unlimited channel doesn't correctly invoke onUndeliveredElement on cancellation Kotlin/kotlinx.coroutines#2435 left)
• introduce SendChannel.safeOffer for better releasing of frames/payloads if channel is closed
• better channel handing in TCP and local transports

Overall: less leaks and better structured concurrency

[OlegDokuka]

LGTM