1.5.1: leave the store to another client's turn

Fixes 1.5.0's lock handover cutting another client's turn short (#38). ATEM Software Control connecting while this package transferred stills lost its thumbnails: the switcher hands the store to a waiting request at a release without announcing it, and 1.5.0's next request was granted beside it and aborted it (FTDE 6). Now the next request waits for the store to be announced free and a frame more, never while another client holds it, and waits while anyone is seen at work. On a 1 M/E Constellation HD, ASC connecting during a back-to-back download loaded all 20 thumbnails (0 with 1.5.0). Uploads alone cost about 0.1 s more per still: 20 take 14.2 s.

New transport API UdpProtocol.wake_at, which the fix uses; nothing removed. MEDIA_LOCK.md: "Sharing the store with ATEM Software Control"; a LOCK sent while another session holds the store is queued, not dropped.