refactor(mongo-client): support a pool minSize option

This is required for testing retryable writes with only a single
connection.

NODE-1105

Author: mbroadst

lib/mongo_client.js

@@ -88,7 +88,8 @@ var validOptionNames = [
   'fsync',
   'readPreferenceTags',
   'numberOfRetries',
-  'auto_reconnect'
+  'auto_reconnect',
+  'minSize'
 ];
 
 var ignoreOptionNames = ['native_parser'];

lib/topologies/replset.js

@@ -75,7 +75,8 @@ var legalOptionNames = [
   'promoteValues',
   'promoteBuffers',
   'maxStalenessSeconds',
-  'promiseLibrary'
+  'promiseLibrary',
+  'minSize'
 ];
 
 /**