Add ratio limiter to AudioStream

If an out-of-bound stretching ratio is about to be used,
(e.g. because of a tempo changing glitch), limit the
stretching ratio to sane values. This will workaround
Windfisch/muse#7.
whitespace
Flush audio prefetch upon tempo changes

This *will* cause glitches, however they will hopefully be
not that bad as without this commit.
Works around Windfisch/muse#7 even more.