[MRESOLVER-346] Too eager locking in resolver (s4u#272)

The locking in resolver is too eager. This PR relaxes locking by introducing use of shared locks (not used before) and following changes:
* installer -- no change, remains exclusive locking
* deployer -- laxed to shared locking, as it only reads local repo
* Artifact and Metadata resolver -- implemented "upgrade", they are optimistic and start with shared lock (and will happily finish if local repo has all) but will "upgrade" to exclusive if remote access (hence local caching) is about to happen.

Best viewed with whitespace ignore, as there are not much change but blocks got nested.

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https://issues.apache.org/jira/browse/MRESOLVER-346