Update protobuf to 2.5.0.
Use the maven plugin.
Do not rely on correct version of protoc being in path. Instead assume source checkout relative to build area.
merge MMcM: Update protobuf to 2.5.0.

Merge pull request #989 from MMcM/sql-layer/protobuf-250
https://github.com/FoundationDB/sql-layer/pull/989