pool: Fix lock problems in xrootd and http movers

When the pool is full, a blocking preallocate call could block
simple 'mover ls' queries and thus cause the entire pool to appear
unresponsive.

Target: trunk
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6795/