Send a test message before committing a new audit device.

[mgritter]

Also, lower timeout on connection attempts in socket device.

Some history here:

1. I first tried just sending a normal message. This doesn't work because the Salt is not yet initialized, and it can't be initialized until the storage view is readable, and it's set to read-only during construction. (Probably for replication-related reasons, although it's not clear whether that is just copying what secret mounts, or is actually necessary here.)
2. Then I tried doing the check in the backend Factory methods, like the file backend does. I thought this would be a bad idea, because those checks would then get re-run every time Vault unsealed and recreated the audit devices. Worse, if they failed, then that audit device would be temporarily removed until Vault went through another unseal-- even if it was weeks.
3. So, I added a new method specifically for testing, which creates a Salt that is used once and discarded.

The test message looks like this:

{"time":"2020-12-09T20:14:22.451152342Z","type":"request","auth":{"token_type":"default"},"request":{"id":"3ca7b3cc-7581-77c7-8610-b80b6284e43f","operation":"update","namespace":{"id":"root"},"path":"sys/audit/test"}}

[swayne275]

Looks good overall! Just a few comments and things I'd like to know.

[swayne275]

should there be a default case? otherwise i'd think writer could be nil?

[mgritter]

writer can be nil, but confusingly for this backend the log function uses open() when it is