Update compat test to set index.search.directory

This probably should not be GLOBAL_OFFLINE in the first place
Update changelog for 0.5.1
Update upgrade instructions for 0.5.1
Update upgrade instructions for 0.5.1

Copied @mbroecheler's UPGRADE.asc additions on
21478ebeea4f5df27b757373b3bf5dc046c34e29 to docs/upgrade.txt, added a
date
Revert CommonsConfiguration to Enum.toString usage

This commit effectively reverts
4c2c34226686ed91feb230af5c8a7a8e39b70403, but it also changes
Timestamps.toString just delegate to the name method, so the strings
stored in CommonsConfigurations backers will still be e.g. MILLI
rather than Timestamps[ms].  Like
4c2c34226686ed91feb230af5c8a7a8e39b70403, this is orthogonal to
KCVSConfiguration which uses a StandardSerializer to convert between
enums instances and StaticBuffers.
Updating titan.compatible.versions prop
[maven-release-plugin] prepare release 0.5.1
[maven-release-plugin] prepare for next development iteration
Add 0.5.1 to compat manifest