Merged refs/pull/6235/head from repository cms-sw
Tuning of the timing, retry policies, and IDs.

Based on further tuning on the global redirector, try to tune
behavior of the algorithm to better perform in the ``real world".
Pipeline multiple readv requests.

Xrootd has a protocol-enforced limit on the number of readv
request chunks a client can issue.  Previously, we chopped the
full request into multiple requests and issued requests / waited for
response one-at-a-time.

With this work, we issue all the requests at once, then block until
all the responses come in.
Improve handling of dCache xrootd hosts.

In this commit:
- Refactor detection of dCache pools.
- When we encounter a dCache pool and we believe that this is a
  federation context (at least 3 xrootd servers), then we try to
  exclude the last xrootd-implemented server on error.
Merge remote branch 'official-cmssw/CMSSW_7_3_DEVEL_X' into xrootd-dcache-tunings
Merged refs/pull/6410/head from repository cms-sw
Touch-ups from code review.
Add explicit chrono header.
Fixes from testing at dCache sites.

Found a corner case where a non-dCache site would be identified as
dCache if XrdCl::File encountered an error at a dCache site and
successfully recovered.

Commit also contains a tweak to the exception generated in XrdFile,
based on Chris's code review.
Merge pull request #6410 from bbockelm/xrootd-dcache-tunings

Xrootd dCache Tunings