The connection pool is creating connections using the supplied factory
function as needed whenever lockConnection is called. Connections are
associated to the calling fiber, as long as any copy of the returned
LockedConnection object still exists. Connections that are not associated
to any fiber will be kept in a pool of open connections for later reuse.
Note that, after retrieving a connection with lockConnection, the caller
has to make sure that the connection is actually physically open and to
reopen it if necessary. The ConnectionPool class has no knowledge of the
internals of the connection objects.