Allow a gap at player format changes

[maximmaxim345]

Servers that share one encoder between players can only switch a player's format at that encoder's next chunk boundary, so they couldn't meet the exact timestamp rule. The server should still continue exactly where the previous format ends, but may now leave a gap. Overlaps and resent audio stay forbidden, and players already fill the gap with silence through the existing sync rules.

Switching the output between formats without adding a gap is now the player's job. Players that can't do that should keep listing a single sample rate and channel count, as before.

aiosendspin already follows these rules since:

• Fix resent audio at mid-stream player format changes aiosendspin#456

Closes #294

• I accept the Contributor License Agreement for this contribution.