Add multi-host connection pooling

Use keyed object pool from Apache Common Pool and a queue for host
names to ensure that connections are chosen on a first-available-
first-used (first-in-first-out) basis. Take a host name from head
of queue when obtaining connection from pool; put host name at tail
of queue when returning connectoin. The queue has n entries for each
host name, equivalent to setting for number of connections per host.
The host names are configured in the OzTrack properties file.