servers: libxtreemfs for Java: Fixed problem that synchronous RPC calls

did not wait between two attemps to execute the request.

A result of the problem was that retry attempts were exhausted very fast
e.g., when the server refused the connection.

This commit adds a simple Thread.sleep() which waits
options.getRetryDelay_s(). An advanced version could take the time
already elapsed into account (like the C++ client does).