add connection timeout option for ES RestClient

This request came in through [janusgraph-users](https://groups.google.com/forum/#!topic/janusgraph-users/Kc-gQs7S9z0)

> Currently, there is no configuration parameter for connection timeout for ElasticSearch requests when Janusgraph is creating a RestClient object. The default value for the connection timeout is 500 ms and is very low for our usage.

References:
* https://www.elastic.co/guide/en/elasticsearch/client/java-rest/current/_timeouts.html
* https://hc.apache.org/httpcomponents-client-ga/httpclient/apidocs/org/apache/http/client/config/RequestConfig.Builder.html
* https://github.com/JanusGraph/janusgraph/blob/v0.3.0/janusgraph-es/src/main/java/org/janusgraph/diskstorage/es/rest/RestClientSetup.java#L82

Start in janusgraph-es/src/main/java/org/janusgraph/diskstorage/es/rest/RestClientSetup.java at line 82, then read the linked Elasticsearch timeout and RequestConfig.Builder references. Trace how Elasticsearch settings are configured and make the connection timeout configurable instead of relying on the 500 ms default; done means the option reaches the RestClient.

Written by the indexing model from the issue text.