[SharedCache] Rework how file accessors are handled

Previously, `MMappedFileAccessor::Open` attempted to impose a fixed limit
on the number of file accessors that were live at one time. This was
done because the default file descriptor limit on some platforms is
relatively low (256 on macOS). Given that recent iOS shared caches can
contain 60+ files it is easy to tie up a large percentage of this limit
by opening one or two shared caches.

This is problematic as if the limit imposed by `MMappedFileAccessor::Open`
is reached, the attempt to access the file will block waiting for
another file accessor to be closed. This can lead to a deadlock.

This commit makes three changes to this strategy:
1. It attempts to raise the file descriptor limit to 1024. Unix systems
   support both soft and hard file descriptor limits. The soft file
   descriptor limit is what is enforced, but a process can explicitly
   raise the limit to any value below the hard limit if it wishes.
2. The fixed limit on open files accessors is changed to a soft limit.
   `FileAccessorCache` is introduced to manage the caching of file
   accessors. It provides a basic LRU cache. Whenever a new file is
   opened, the cache of open accessors is pruned to stay below the
   target limit (50% of the soft file descriptor limit). This limiting
   is primarily done to allow files containing dirty pages to be
   unmapped if they're no longer being used.

   LRU isn't the optimal strategy for this, but it is simple to
   implement and understand. Ideally there'd be some access time
   component to the cache so files that haven't been accessed can be
   released.
3. File accessors are cached even for sessions corresponding to views
   that have been closed. The "Open Selection with Options..." context
   menu hits this case as a view is created and destroyed as part of
   populating the options dialog, and the real view is then created in
   the same session.

The most significant benefit of this change is that the logic around
opening / closing file accessors is simpler and can no longer deadlock.

While working on this I noticed that `SharedCache` opened some files via
`MMappedFileAccessor::Open` without specifying a post-open operation to
apply slide information. Instead, it would explicitly apply the slide
information after opening the file. This works fine so long as the file
accessor limit is never hit. If it is hit and the accessor is closed,
the next time the file is opened it will not have any slide information
applied. This would lead to very confusing bugs.

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