Change xid computation logic

[lobachpavel]

Zookeeper doesn't handle xid = 0 properly, so change logic to start xid from 1 and properly handle integer overflow

see #100 for detailed explanation

[rambo-panda]

I think we just need to set the initial value of XID to 1.

because
a: The role of xid is to ensure the order of requests, so we should not break its meaning.
b: 2 ** 32 -1 I believe this value can meet most needs

[dreusel]

With a continuously running application doing on average 100 requests per second (mine, which is not a huge one, already does way more), that would break after 248.5 days.