Idea: Purge OTC only after first upload by a keypair

[burke]

Currently we purge one-time-codes as soon as the KeyClaimRequest is processed, which means that if the KeyClaimResponse is lost, the user will need to ask their HCP for a new code.

In order to mitigate this, we should consider purging the OTC only as the first upload is being recorded, so that KeyClaimRequest can be idempotent if the same keypair and OTC is received again as a duplicate request before the first upload.

(thanks @dylanahsmith for pointing this out)