FileStore: _split_collection should not create the collection

This will simplify adding a replay guard to create_collection.

Backport: bobtail
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit b184ff581a08c9e6ce5b858f06ccbe9d0e2a170b)
FileStore: set replay guard on create_collection

This should prevent sequences like:

rmcoll a
mkcoll a
touch a foo
<crash>

from causing trouble by preventing the rmcoll
and mkcoll from being replayed.

Fixes: 4064
Backport: bobtail
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 411770c45734c9827745ddc4018d86c14f2858a6)
FileStore: fix reversed collection_empty return value

Backport: bobtail
Fixes: #4380
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
(cherry picked from commit 7a434d10da0f77e1b329de0b06b6645cd73cc81b)

Conflicts:
	src/os/FileStore.cc