reviewers needed for "HADOOP-17752. Remove lock contention in REGISTRY of Configuration "

[Steve Loughran <st...@cloudera.com.INVALID>]

Can some people who understand weak references &c take a look at this PR:
https://github.com/apache/hadoop/pull/3085

It's aim is to remove a lock in Configuration construction, so could speed
construction up. It does get fairly complicated though, which is why it
needs many eyeballs.

We've hit scale issues with Configuration in the recent past (different
hive threads creating FS instances). I don't think this was where the
problems surfaced, but given how broadly Configuration is used: (a) better
scale is good and (b) we don't want to break it

-steve