cluster: update cluster state when disconnecting a node.
cursor: use batch size for n_return.

Limit actually belongs with $query, or negative value to exhaust. Needs
more investigation.
tests: use w=3 write concern when inserting documents.
tests: more assertions on cluster and cursor state.
trace: add tracing macros.
tests: link against private library.
log: add trace log level.
cluster: use mongoc_client_t instead of void*.
tests: print write concern to console.
tests: more assertions.
cluster: disconnect after counter increment.
cluster: be more careful about tracking replicaset state.
log: cleanup default logger.