Check in the page grouping prototype

[geoffxy]

All the code is self contained under the page_grouping subdirectory and is under the llsm::pg namespace so that it does not interfere with the existing code. There are also some new experiment configurations that I put under scripts/pg.

A general overview of what is where:

• debug: Code for standalone executables that help with debugging. Useful to keep around in case we need it later on.
• persist: Code related to reading/writing pages to/from disk. This directory contains adapted copies of some existing code in LLSM (the page, packed map, physical page ID (now segment ID), file (now segment file)).
• plr: Implementation of piecewise linear regression
• tests: Tests specifically for the page grouping prototype
• tools: Contains the "page shuffling" tool we need for our benchmarks
• manager.h and manager_*.cc: The primary read, scan, write logic
• segment_builder.h/.cc: The logic for building segments using the piecewise linear regression code

The prototype relies on some dependencies outside page_grouping (the TLX B+Tree, some utils, and the page memory allocator under bufmgr).

There's a lot of code; I have not added many comments because the interfaces may change. I'd say the most important files to look over are the manager related files.

cc @andreaskipf @mmarkakis

[geoffxy]

I'll fix the failing build. Not sure why this builds fine locally.