fix resending of rate-limited messages

Context

when resending rate-limited messages:

• sb experiences outages due to a deserialization error we thought we addressed in !168 (merged), but the deserialization error is still happening and we still have outages
• messages are resent at a rate that seems likely to violate the leaky bucket algorithm signal uses for rate limiting (which allows ~60 messages/min)

To fix:

• prevent deserialization errors
• increase resend interval to be more likely to conform with signal rules (and lower number of allowed tries)

out of scope

• append a timestamp-y thing "send N minutes ago" for resent messages (since messages could grow quite stale with a 1 min ^ power of 2 interval)