Preserve osm changeset close time as commit's timestamp
Upgrade maven surefire plugin and parallel test execution settings.

Parallel test execution settings were using the deprecated 'forkMode'. Replaced
by forkCount/reuseForks as recommended by the plugin's documentation.
Add missing 'final' modifier to RevTree.EMPTY and RevTree.EMPTY_TREE_ID.
Reorganize CLI functional tests into general, plumbing, porcelain, and remote.

Helps reduce the build time by allowing maven to run the tests
for the different packages in parallel.
Avoid forks for the mongo tests, they all hit the same db
Avoid forks for the oracle and sqlserver tests, they all hit the same db