Renewed certs are not used

[cdunklau]

This is not a bug in rproxy per se, it lies with txsni. I'm posting it here to keep rproxy users in the loop.

While the txacme AcmeIssuingService successfully renews certs, the new certs aren't used by rproxy. This is reported in https://github.com/glyph/txsni/issues/12 and fixed in https://github.com/glyph/txsni/pull/22 ~~but that fix hasn't made it to a release yet~~ and should be in the v0.2.0 release. Restarting rproxy works around this.