NEXUS-5930: removing method from interface

As this is internal concern of shadow repositories,
is not part and should not be part of their public contract.
NEXUS-5930: Marking method as thread safe

Event processing still happens synchronously (so no
ordering problem), but multiple events might be processed
simultaneously.
NEXUS-5930: Upping OBR logic to recreate md out of read lock

Changed which method is overridden, to make sure that
request corresponding path's UID shared lock is released
once retrieval is attempted, and before the OBR
md recreation attempt happens to avoid deadlock.
Merge pull request #191 from sonatype/NEXUS-5930-obr-deadlock

NEXUS-5930: OBR shadow makes Nexus deadlock prone