circe-tls-try-single-command: Also add a process sentinel.

Emacs does not guarantee a process filter to run if the process
fails without emitting any output. Use a sentinel to check for
connection failures.
Fixed some of circe-tls issues

Added a check for process attached to a target buffer, so now it must not
assign the same buffer to different processes, even if they are running
simultaneously.

And removed a call to fail-thunk from the temporary process filter, so now
only it's sentinel calls the fail-thunk (and it does not open excessive
connections on error).
Merge pull request #76 from defanor/master

Fixed some of circe-tls issues