Add dedicated executor to close connection in NIO mode

[acogoluegnes]

When sharing the same executor for NIO and connection closing, all the
threads of the pool can be busy recovering connections, leaving no
thread left for IO. This commit add a new executor service to the NIO
mode to submit all the connection closing to.

This is useful when an application maintains dozens or hundreds of
connections and suffers massive connection lost. Hundreds of connection
closing tasks can be submitted very quickly, so controlling the number
of threads and leaving some threads available for IO is critical.

If an application maintain just a few connections and can deal with the
creation of a few threads, using the new executor isn't necessary.

Fixes #380