Override Map.merge() default method for correctness and efficiency. Partially addresses #500

[mohrezaei]

Just thinking this over:

• some collision tests (chained entries) would improve coverage for Unfied*
• if the result of calling the function is oldValue (i.e. newValue == oldValue) a lot of processing can be skipped.

[mohrezaei]

additional things to do from the javadoc:

1) If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged. 2) In particular, all implementations of subinterface ConcurrentMap must document whether the remapping function is applied once atomically only if the value is not present.

for (2), we have to document that all these are possible:

• function is called multiple times
• function is called one or more times, but the result is not used (because the old entry was concurrently removed)

Also I think the javadoc language for (2) is broken. "applied once atomically only if the value is not present. " is not sensible: if it's not present, it shouldn't have been applied, no?

for (1), we can't leave the map in an intermediate state. I don't see any such case, but a test or two would be good.