pool: Fix problems with restoring permanent migration jobs

Addresses a regression introduced in 2.6 in how migration jobs are
restored on pool startup. The issue was that restored migration jobs
would have "null" as the command string, which would show up in
migration ls and migration info output, as well as stored back to
the pool setup file when the save command was used. In the latter
case, a subsequent pool restart would fail.

Target: trunk
Request: 2.6
Require-notes: yes
Require-book: no
Fix regression in message queue length reporting

Addresses the issue that the message queue length in cells
is always reported as zero. This regression was introduced
between dCache 1.9.5 and 1.9.12.

Target: trunk
Request: 2.6
Request: 2.2
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/5875/