rekorv2: parallelize entry requests

[jku]

This is followup to #1432

• Since rekor v2 responds to the request only after the inclusion proof is available, signing multiple artifacts is slow with the way the client is currently implemented (each entry ends up integrated into the log in a separate batch, and each of those takes a few seconds)
• if we instead sent the rekor requests in parallel, all of them could be integrated in one go so signing should not be very slow
• this might be harder than expected -- the architecture is not designed for this and e.g. requests Session is not thread safe

(alternative to parallelizing is to talk rekor-tiles project into providing api for creating multiple entries with one call... But that would still require client changes)

Originally posted by @jku in #1432 (comment)