Allow for no retry jitter

[bblanke]

There needs to be a way to eliminate retry jitter. For my use case, I need the autobahn client to attempt to reconnect to a server every 2 seconds. With the pseudo-random retry times introduced by the jitter, my retry time can end up anywhere from a few milliseconds to 30 seconds (depending on how long I leave it). That's not ideal for me.

The code already has a clause that only applies retry jitter if the _retry_delay_jitter property is truthy. So if the property were 0, then no retry jitter would be applied.

However, due to the way self._retry_delay_jitter is initialized, it will never accept a value of 0. This pull request changes that and allows self._retry_delay_jitter to be set to 0.