Expose vibe.d connection pool's removeUnused function

[schveiguy]

Every time my vibe.d server shuts down, I get a bunch of messages such as:

Warning (thread: main): leaking eventcore driver because there are still active handles
   FD 7 (streamSocket)
   FD 13 (streamSocket)
   FD 14 (streamSocket)

These are due to open MySQL connection pool connections. I want to get rid of these, and vibe.d included the nice feature to close all connections not in use here.

But I can't use it, because m_pool is private in MySQLPool. This functionality should be exposed.

[schveiguy]

PR: #210

[Abscissa]

#210 is merged