Do not use the recently introduced instance IDs in the cache keys (as per
discussion in http://svn.haxx.se/dev/archive-2014-08/0239.shtml).  Utilize
them only when accessing the filesystem shared data.

We were trying to avoid stale cache entries in a situation when a repository
is hot swapped with an older version of itself.  However, doing so might give
the users a false sense of security, as long as the instance IDs do not work
on copy-pasted or snapshotted repositories.  Users are unlikely to actually
corrupt the repository with or without the instance IDs (the race is quite
hard to hit, see below).  However, *without* the instance IDs in the cache
keys, they are going to receive different sort of errors both on the client
and on the server, e.g.:

  E200014: Checksum mismatch for 'path'
  E200014: Checksum mismatch while reading representation
  E160004: Malformed svndiff data in representation"

These errors should give the users a hint that what they're doing is wrong
and will vanish after the server restart.  We consider this a better choice
compared to silently narrowing the window of the problem.

A bit more on the possible repository corruption — the race is triggerable
with the following sequence of actions.  The introduction of the instance IDs
does not have an ability to solve the problem, but I suppose there might be
another way (and I am probably going to track this issue separately):

- We start with an empty repository served by Apache HTTP Server
- Then we create a backup of the empty repository with 'cp repository'
- svn checkout [wc1]; echo A > file ; svn add file ; svn commit
- Now we perform a hot swap of the backup (with 0 revisions)
- svn checkout [wc2]; echo B > file ; svn add file ; svn commit
- Now, within [wc1] — svn cp file file-copy ; svn commit

  (this will corrupt the repository with 'svn diff -c 2' returning a
   "svn: E200014: Checksum mismatch for 'file" error)

* subversion/libsvn_fs_fs/caching.c
  (svn_fs_fs__initialize_caches, svn_fs_fs__initialize_txn_caches):
   Do not use the instance IDs in the cache keys.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1623402 13f79535-47bb-0310-9956-ffa450edef68