Support OpenSRF+Redis for Evergreen 3.12

[Bill Erickson]

Spawned from bug #2017941

Bug for tracking Evergreen 3.12 patches to better support installing Evergreen 3.12 atop OpenSRF+Redis for new installations.

[Bill Erickson (berick)]

https://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/berick/lp2041431-opensrf-on-redis-v3-eg3.12

* Adds commented-out Redis config options to opensrf_core.xml.example
* Docs for switching EG configs to Redis

Note these changes are only needed for EG 3.12 where it's assumed that Ejabberd is the default, but Redis is supported. Post-3.12, where Redis is the default OpenSRF variant for new installations, the configs in bug #2017941 will just work. In theory, we don't need multi-flavor instructions (Redis vs. Ejabberd) for new EG installs post EG 3.12, but they can be added if needed.

[Bill Erickson (berick)]

Added pullrequest, but note these changes should not be merged to 3.12 until a) we have a 3.12 branch and b) we have merged bug #2017941.

Alternatively, we could merge these along with bug #2017941 and revert/modify them for EG main.

[Jane Sandberg (sandbergja)]

Thanks, Bill! These are the first two commits merged to rel_3_12. :-D