pool: Fix logging context for thread pool

Fixes the logging context in a thread pool used by migration tasks,
pool to pool tasks, and sticky bit expiration.

Target: trunk
Request: 2.8
Request: 2.7
Request: 2.6
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6675/
(cherry picked from commit 4f79cb3dbd2e7662da7f73dacae68a8f129b5101)
Merge pull request #450 from gbehrmann/fix/2.7/rb6675

pool: Fix logging context for thread pool