Enable client-side load-balancing in XrplClient.java

[sappenin]

The idea here is that the current s1.ripple.com setup uses a DNS TTL of 5s, so any networking stack will peg the XRPL4j client to a given IP address for at least that long (or whatever the DNS TTL happens to be). If the node that the client stack is pegged to (i.e., the IP Address) is down, or not accessible, then the xrpl4j client will lose access.

One alternative here, especially in private-ledger scenarios (e.g.) where the xrpl4j client know which IP addresses to be talking to when making rippled calls, would be to allow the client to be constructed with one or more URLs and/or IPs so that if the client is unable to communicate with the rippled host, it can attempt to use a different host without waiting for DNS timeouts that the client cannot control.