lib-smtp: client: Move connection to TRANSACTION state immediately when a transaction...

lib-smtp: client: Move connection to TRANSACTION state immediately when a transaction is started.

Before, it waited until the actual start handler was called from timeout, but
that causes a race condition with mtp_client_connection_abort_transaction(),
which in turn causes an assert panic.

Panic was:

Panic: file smtp-client-connection.c: line 2153 (smtp_client_connection_abort_transaction): assertion failed: (conn->state != SMTP_CLIENT_CONNECTION_STATE_READY)