Fix concurrent initialization of MonetaryRoundings

In a container or parallel test environment the initialization of MonetaryRoundings fails sporadically.

This fixes <https://github.com/JavaMoney/jsr354-ri/issues/30>, the issue was actually in the non-threadsafe usage of a `ConcurrentMap` inside `DefaultServiceProvider` in the api project and not the in the reference implementation.

This pull request is based on the 0.8 tag, since I'm currently still on java 7.