CDRIVER-2558 add "snapshot" readConcern

Author: ajdavis

src/libmongoc/doc/mongoc_read_concern_t.rst

@@ -30,6 +30,7 @@ MONGOC_READ_CONCERN_LEVEL_LOCAL         Level "local", the default. 3.2
 MONGOC_READ_CONCERN_LEVEL_MAJORITY      Level "majority".           3.2
 MONGOC_READ_CONCERN_LEVEL_LINEARIZABLE  Level "linearizable".       3.4
 MONGOC_READ_CONCERN_LEVEL_AVAILABLE     Level "available".          3.6
+MONGOC_READ_CONCERN_LEVEL_SNAPSHOT      Level "snapshot".           4.0
 ======================================  =========================== =====================
 
 For the sake of compatibility with future versions of MongoDB, :symbol:`mongoc_read_concern_set_level` allows any string, not just this list of known read concern levels.

src/libmongoc/src/mongoc/mongoc-read-concern.h

@@ -32,6 +32,7 @@ BSON_BEGIN_DECLS
 #define MONGOC_READ_CONCERN_LEVEL_LOCAL "local"
 #define MONGOC_READ_CONCERN_LEVEL_MAJORITY "majority"
 #define MONGOC_READ_CONCERN_LEVEL_LINEARIZABLE "linearizable"
+#define MONGOC_READ_CONCERN_LEVEL_SNAPSHOT "snapshot"
 
 typedef struct _mongoc_read_concern_t mongoc_read_concern_t;