DATAREDIS-271 - Add support for 'pSetEx'.

Support for 'pSetEx' has been added for 'jedis' & 'srp'.
For 'lettuce' the command is emulated using 'eval'.

There is no support for 'pSetEx' when using 'jredis'.

'DefaultValueOperations.set' used with 'TimeUnit.MILLISECONDS' will fallback to 'setEx' in case 'pSetEx' is not supported by the driver in use.

Original pull request: #46.