Performance fixes for EncryptionMessageMutator

Fix for issue #1630 - Performance improvements in reflection code by
caching in some strategic places. This improves performance of this
mutator by ~10x.
XmlMessageSerializer uses DelegateFactory cache

Made XmlMessageSerializer rely on delegate cache in DelegateFactory.

I put this in a separate commit as to isolate the change in
XmlMessageSerializer (and any regression that might come out of that).
Removed ConcurrentDictionary and perf unit test

After code review, replaced ConcurrentDictionary with regular
Dictionary, and removed performance unit test so that it can be covered
in PerformanceTests.
Added performance test and fixed resulting bugs

Added performance test for the EncryptionMessageMutator and fixed bugs
resulting from running the performance test.
Merge pull request #1745 from roycornelissen/issue_1630

Performance improvements EncryptionMessageMutator