Adjust POMs to allow building just a subproject.
merge MMcM: Adjust POMs to allow building just a subproject.

Merge pull request #1515 from MMcM/sql-layer/maven-can-build-subproject
https://github.com/FoundationDB/sql-layer/pull/1515
Change parts of FDB services from private to protected to allow specialization for testing.
Introduce a separate method for the top-level transaction context, defaulting to the database itself.