support 9.3 by allowing disabling replication slots

[zalando]

support 9.3 by allowing disabling replication slots

[jberkus]

Taken care of with PR #23

[CyberDem0n]

Oh!
We can actually make this feature more smart and even make patroni compatible with postgresql < 9.3

1. We should query major version in Postgresql.__init__
2. If version < 9.4 forcefully set use_slots to False
3. Get rid from pg_xlog_location_diff which exists only from 9.3 and use function utils.lsn_to_bytes (https://github.com/zalando/patroni/blob/master/helpers/utils.py#L45) instead.
4. Profit

[CyberDem0n]

Hmm, for postgres version < 9.3 formula is a little bit different...
Ok, will move this method into Postgresql and make it version-specific.

[CyberDem0n]

2 wont work for slaves...
We have to know in advance that version < 9.4 in order to create proper recovery.conf but without starting cluster it's not really possible to determine version.

But at least everything else is doable.

#25

[jberkus]

We don't care about supporting < 9.3 Here's why:

1. Before 9.3, you couldn't remaster over streaming replication, making failover/failback and master change much more complicated.
2. 9.3 was 2 years ago, and is the supported version in current stable Ubuntu, Debian, and Red Hat.

[CyberDem0n]

Ok, dropped unneeded code.