SERVER-7862 Added configurable socketTimeout option to mongos.

Although DBClient* family classes support custom socket timeouts, mongos does not specify any. This can lead to hung queries in cases if mongos is running locally but mongod is running on remote host and a major networking issue takes place.

(associated Jira ticket: https://jira.mongodb.org/browse/SERVER-7862)

This patch adds a `--socketTimeout` opiton to mongos in a manner like `--localThreshold` option is handled.