counters: store counters in shared memory segment.

This includes a header that can be used to navigate the counters.
client: add counters.
cursor: add counters.
mongoc-stat: add tool to dump counters from libmongoc-1.0 based application.

libmongoc-1.0 creates a new shared memory segment that can be used to
dump application counters. This tool loads up the segment for a particular
process and dumps the current state of the counters.