DNB, os/bluestore: minimize size of critical section in omap_iterate()

DNB, os/bluestore: minimize size of critical section in omap_iterate()

This commit takes out both seeing and iterating over onode's
OMAP out of the `Collection::lock`.

It bases on work by Adam Kupczyk on the behavior of RocksDB::DBIter
maintaning consistent view despite ongoing writes. Kudos to Adam.

This is experimental commit; I'm adding it for testing early.
DNB -- Do Not Backport.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>