Reintroduce capped collections backend

There are three commits here:
1. Re-introduce the capped collections back end work
2. Un-refactor the insert executor so it looks more like what's currently on master, except for adding execution of the purge fragment.  Hopefully this will address the performance issue that seemed to be caused the first time I merged to master.
3. Get rid of unused member variable `m_partitionColumnIsString` from insert and update executors.

The first commit has already been reviewed by Xin when I merged it a few weeks ago.  Unreviewed changes are in these files:
* src/ee/executors/insertexecutor.cpp
* src/ee/executors/insertexecutor.h
* src/ee/executors/updateexecutor.cpp