srm/transfermanager: make errors less noisy

If a third-party transfer fails, it gets logged in the pool,
transfermanager general log, transfermanager activity log and
the SRM.  The log messages are also needlessly long and include
unnecessary information.

This patch quietens this considerably.

Target: master
Patch: http://rb.dcache.org/r/6886/
Acked-by: Tigran Mkrtchyan
SRM: add support for 3rd-party HTTP copy

This patch adds support for pulling or pushing data to a remote server via HTTP.
pool: switch to using HttpClient