Revision 1575319

Introduce a subpool in the ra_serf open session handling, to reduce memory
usage in the session pool and to work around a bug in ra serf, where it
doesn't remember to tell us that a request failed/was cancelled.

* subversion/libsvn_ra_serf/options.c
  (svn_ra_serf__exchange_capabilities): Use result, scratch pool.

* subversion/libsvn_ra_serf/ra_serf.h
  (svn_ra_serf__exchange_capabilities): Use result, scratch pool.

* subversion/libsvn_ra_serf/serf.c
  (svn_ra_serf__open): Introduce a subpool. Update caller.