Improve Thrift Transport pool

Accumulo has a pool of recently opened connections to tablet servers.  When connecting to tablet servers, this pool is checked first. The pool is built around a map of list.  There are two problems with this pool :

• It has global lock around the map of list
• When trying to find a connection it does a linear search for a non reserved connection (this is per tablet server)

Could possibly move to a model of having a list of unreserved connections and a set of reserved connections per tablet server. Then to get a connection, could remove from the unreserved list and add to the reserved set.  This would be a constant time operation.

For the locking, could move to a model of using a concurrent map and locking per tserver instead of locking the entire map.