Four very distinct commits here - overall, we now cope with out-of-order system-times from the tx-log by recording the maximum of the time received from the log and the previous tx-time + 1µs.

The main user-visible impact of this is that submit-tx no longer returns the whole transaction key, because the system-time of the transaction isn't finalised until the transaction is indexed - users that require the system-time should either call execute-tx instead, or look up the transaction in the xt.txs table.

This also brings another couple of breaking changes:

• query options: after-tx (taking the tx) -> after-tx-id (taking just the transaction ID)
• at-tx no longer defaults after-tx-id if the latter is not provided, nor does XTDB implicitly await the at-tx - if there is a chance that the transaction passed to at-tx hasn't been indexed yet, users should additionally provide after-tx-id
• loosely related: the :basis arguments have been moved out of their nested :basis map to the top-level of the query options. The (my) previous argument that these are the only options that affect query repeatability isn't correct - default-tz also does, as well as (more bluntly!) flags like :explain?.

resolves #3864