Webdis used to call fsync after every single log message, which had a
significant negative impact on performance. This change introduces 3
config options for fsync: no explicit fsync (the new default), a periodic
fsync called every N milliseconds, or the old behavior.
The new config key is also documented and validates its inputs.
* Change redis_auth in struct conf to handle old and new auth
* Update cfg.c to understand an array of two strings for redis_auth
* Update pool.c to send both username and password
* Add links to new images on AWS ECR
* Clean up Markdown:
* Change all <pre>...</pre> to ``` blocks
* Add syntax info on code blocks
* Add `$` prefix in front of all commands that didn't have it
* Heading tweak