SERVER-10868 Step 1: Make an RAII TempDir class for unit tests

Prep for SERVER-10868: move external sort tests out of dbtests
SERVER-10868 Step 3: Pass the temporary directory into Sorter

This removes the direct link-time dependency on dbpath from Sorter and
the aggregation DocumentSources that use it.

After this commit dbpath can once-again be removed from mongos. Not
making that change now however to avoid colliding with Shaun's work.

Prep for SERVER-10868: move external sort tests out of dbtests
SERVER-10868 Move Sorter tests out of dbtests to a unit test