Add "are indexes in sync" query

[KyleMaas]

Based on #301

This adds a function to sbot which allows for querying whether or not the indexes are all up-to-date at the present time. This could be used for:

• Making sure that publishing during indexing doesn't fork your feed (see Possible race condition in the publish log causing a forked feed #292, Publishing needs to be fully synchronous #293)
• Status indicator in a UI layer above sbot
• Providing some kind of feedback to a user that if they choose to publish, it will not actually be published until after the indexes finish, depending on how Publishing needs to be fully synchronous #293 is implemented

[decentral1se]

Fantastic! Yeh, as promised, the #301 fix really set this on up nicely 😍

[decentral1se]

@KyleMaas #301 has landed and the build is green again (luigi upgrade also landed), rebase away 💃

[KyleMaas]

@decentral1se

Rebased.

[KyleMaas]

Thanks!