xrootd4j: Add kXR_locate support

Implements kXR_locate request and response, and adds an implementation
for the request to the standalone server.

Target: master, 1.2
Merge commit 'd1562f486636aaa740ff204a64ad8ea4581bea4d' into release/1.2

Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5872/