(PDB-880) Errors on PDB shutdown

This patch bumps activemq to 5.7.0.  This seems to prevent two errors that
would sometimes occur on shutdown, one where the broker was stopped before its consumers
were removed and another where KahaDB batching fails to reset due to the PageFile
not being loaded.
Merge pull request #1137 from wkalt/ticket/master/pdb-880-shutdown-exceptions

(PDB-880) Errors on PDB shutdown
(maint) Allow factset hashes to be nil in db

This is necessary for factsets to function after the PDB-898 migration,
since existing factsets are not being hashed.