Added memory caching to lldb_private::Process. All lldb_private::Process

subclasses will automatically be able to take advantage of caching. The
cache line size is set to 512 by default.

This greatly speeds up stack backtraces on MacOSX when using the 
ProcessGDBRemote process plug-in since only about 6300 packets per second
can be sent.

Initial speedups show:

Prior to caching: 10,000 stack frames took 5.2 seconds
After caching: 10,000 stack frames in 240 ms!

About a 20x speedup!

llvm-svn: 122996