git-svn: clean up caching of SVN::Ra functions

git-svn: clean up caching of SVN::Ra functions

This patch was originally intended to make the Perl GC more
sensitive to the SVN::Pool objects and not accidentally clean
them up when they shouldn't be (causing segfaults).  That didn't
work, but this patch makes the code a bit cleaner regardless

Put our caches for get_dir and check_path calls directly into
the SVN::Ra object so they auto-expire when it is destroyed.

dirents returned by get_dir() no longer needs the pool object
stored persistently along with the cache data, as they'll be
converted to native Perl hash references.

Since calling rev_proplist repeatedly per-revision is no longer
needed in git-svn, we do not cache calls to it.

Signed-off-by: Eric Wong <normalperson@yhbt.net>