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.
NEXUS-5944: Extract "page" utility from AR

AR has a bunch of utilities, one of the is Page,
that offers higher (read simpler) level of operations
against remote HTTP servers.
NEXUS-5944: Handle when remote is Nx repo with browsing disabled

Using Page utility to figure out what remote is.
Add IT
NEXUS-5944: Obey truststore
NEXUS-5944: No, you are wrong

These IT classes were NOT copied from SP ITs.
Not a chance. Look how method names differ!
Merge pull request #191 from sonatype/NEXUS-5930-obr-deadlock

NEXUS-5930: OBR shadow makes Nexus deadlock prone
Merge remote-tracking branch 'origin/master' into NEXUS-5944-no-browsing-allowed
NEXUS-5944: Fix IT

Not using central but rather releases as central indexes
caused timeout in IT
Merge pull request #192 from sonatype/NEXUS-5944-no-browsing-allowed

NEXUS-5944: Repository is auto-blocked if "allow file browsing" is disabled on remote
Merge remote-tracking branch 'origin/master' into NEXUS-5993-files