features/shard: Introducing ".shard/.remove_me" for atomic shard deletion (part 1)

PROBLEM:
Shards are deleted synchronously when a sharded file is unlinked or
when a sharded file participating as the dst in a rename() is going to
be replaced. The problem with this approach is it makes the operation
really slow, sometimes causing the application to time out, especially
with large files.

SOLUTION:
To make this operation atomic, we introduce a ".remove_me" directory.
Now renames and unlinks will simply involve two steps:
1. creating an empty file under .remove_me named after the gfid of the file
participating in unlink/rename
2. carrying out the actual rename/unlink
A synctask is created (more on that in part 2) to scan this directory
after every unlink/rename operation (or upon a volume mount) and clean
up all shards associated with it. All of this happens in the background.
The task takes care to delete the shards associated with the gfid in
.remove_me only if this gfid doesn't exist in backend, ensuring that the
file was successfully renamed/unlinked and its shards can be discarded now
safely.

Change-Id: Ia1d238b721a3e99f951a73abbe199e4245f51a3a
updates: bz#1568521
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>

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