Always set BSON_MEMCHECK for valgrind and ASAN tests, and add replica
set auth tests to the valgrind, ASAN, and code-coverage matrixes. Reduce
the number of special rules and exceptions in the matrix.

Add bson_iter_overwrite_date_time, bson_iter_overwrite_oid, and
bson_iter_overwrite_timestamp.

Also, start testing MongoDB 4.0 on zSeries, CDRIVER-2686.

mongoc_apm_server_heartbeat_succeeded_t, mongoc_apm_server_heartbeat_failed_t
duration fields had been uninitialized. Test that they are set, and the same
command monitoring structs (which were implemented correctly).

All mongoc headers are under mongoc/ and all bson headers are under
bson/. The preferred way of including the headers are mongoc/mongoc.h
and bson/bson.h respectively. Forwarding headers in the root are
provided for backwards compatibility.

Preparation work to be able to create darwin style frameworks.