pool: Resolve high memory usage and other issues in sweeper

Space sweeper while reclaiming loads the meta data entry of the
files it will remove. This consumes a lot of memory, in particular
when the entire pool is purged, causing OOM in the worst case.

This patch resolves this issue. The patch also resolves an issue
with the 'sweeper ls' command which would block the message thread.

Finally, log messages indicating the end of long running operations
in sweeper and for 'rep rmclass' have been added. This allows the
admin to detect when these operations have finished. Other log messages
have been refined too, including lowering the log level of several
messages.

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7195/
(cherry picked from commit 467ad69ceaa2d5f7cdb4dd8ce21f36015381bdf4)
Merge pull request #766 from gbehrmann/fix/2.10/rb7195

pool: Resolve high memory usage and other issues in sweeper