webdav: Fix certificate handling for HTTP third-party copy

The third-party copy support in the webdav manually extracts the certificate
chain from the request. It uses GLite VOMS validator component to extract the
primary FQAN. The VomsValidator requires a local vomsdir configuration, thus
requiring replicating configuration we already got in gPlazma.

Since the CopyFilter that implements third-party copy is invoked after the
SecurityFilter that communicates with gPlazma, there should be no reason why
CopyFilter couldn't simply access the current Subject (as returned by gPlazma)
and extract the DN and FQAN from it. This avois having a vomsdir setup on the
webdav door. This change is implemented by this patch.

The patch also updates the capath configuration in the mover used for
third-party copy. The mover would expand the *entire* dCache configuration to
subsequently extract a single property. It would do this on every transfer.
This is updated to only expand the single property that is needed, thus
reducing the per transfer overhead.

The patch fixes a synchronization issue in the mover in which the loaded CA
certs are stored in a static TrustStore, however access of that TrustStore was
synchronized locally on the mover instance, not on something shared by the
movers (like the mover class).

The patch fixes an issue caused by using per pool CA directories. Since the
mover cached this once, all pools of a domain would use the CA directory
configured for the first mover started. This is solved by using a Guava
LoadingCache. The cache also solves another problem: That of not reloading
the certificates. This is now achieved by expiring cache entries after
one minute.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: https://rb.dcache.org/r/7171/
(cherry picked from commit deb59961e3a9e89b94724b8c4935cfa14d02b92a)
Merge pull request #718 from gbehrmann/fix/2.10/rb7171

webdav: Fix certificate handling for HTTP third-party copy