Let's do a 1.9.1 release

[Christopher]

Hi Devs,

I'd like to do a 1.9.1 release to fix the recently discovered write-ahead
log issues, some of which are critical:

https://github.com/apache/accumulo/issues?utf8=%E2%9C%93&q=label%3Av1.9.1

I can start a RC later today or tomorrow.