Get rid of static executor service

[acogoluegnes]

See #80 (comment)

[DimaGolomozy]

ChannelHandler also uses this executor,
But I think the problem can be solved when close/abort are invoked.
If they are invoked from ConnectionHandler class, then the executor should be stopped.

[acogoluegnes]

The static executor service scopes all the connections created in the process. Stopping it for a connection would make the recovery impossible for all the other connections created in the process.