* Terminal host: LocatorCallback now returns an error

Allows for easier Run() termination in case a user-provided callback
fails.

* TerminalServer: simplify findTerminal()'s map query

* TerminalChannel: always populate errChan and return after it's done

* requestFromGuest should really be requestFromHost

* helloFromGuest should really be helloFromHost

* TerminalServer: simplify FindSession()'s query

* TerminalServer: return nil on channel.Context().Done()

* DataChannel: return more appropriate error on session closure

* TerminalHost: clarify weird handling of Recv() error

* launchDataChannel: bail out if we've failed to create PTY

* ioToPty + ioFromPty: only update last activity after reading the data

* Introduce TestTerminalDimensionsToPtyWinsize

* Renabe subCtx to dataChannelCtx for clarity's sake

* TerminalHost: consistent handling of data channel's context termination

* TerminalHost: provide missing methods on Windows

* Added a Dockerfile to produce an image from Cloud Build

* Added a data flow diagram and improved README.md

* TerminalHost: only update last activity on PTY input

* README.md: swap host and guest in the list of system components

* README.md: swap "host" and "guest" in server definition

Co-authored-by: Fedor Korotkov <fedor@cirruslabs.org>

* README.md: reoder components list, place server in the middle

Co-authored-by: Fedor Korotkov <fedor@cirruslabs.org>