Support for cursors in crud metadata
get_*_info -> find_*
Check for closed sockets before attempting a rpc

Ensure that the socket we've selected has been read from within a
second.  If it hasn't, check if it's closed by polling for readability
and if that works, MSG_PEEK'ing a single byte.  If we get eof, or an
error, we can assume it's broken.  This lets us avoid a large class of
transient network failures.

Closes #136
Merge branch 'mongodb-master'

Conflicts:
	src/mongoc/mongoc-cursor-array.c
	src/mongoc/mongoc-socket.c
	src/mongoc/mongoc-socket.h