tidy up early-data state-machine interactions

[kaduk]

Promoting my comment at quictls/openssl#8 (comment) to a more discoverable issue:
When we successfully return from the handshake as a client, we have to check if the early-data state was in SSL_EARLY_DATA_CONNECTING and set it to a new value to indicate that we can no longer send early data (since we got handshake data from the server and should send with 1-RTT keys). The current code sets SSL_EARLY_DATA_WRITE_RETRY, but IMO SSL_EARLY_DATA_FINISHED_WRITING is more appropriate; ossl_statem_check_finish_init() would translate the former into the latter eventually anyway, IIUC.