Added streamstress.c stress test for xinsert streaming

Added streamstress.c, which sets up a number of voices with xinsert
units and uses a2_OpenSend() and a2_Write() to asynchronously render
and stream audio to them.

More descriptive A2_NOUNITS error code that a2_XinsertAddClient()
returns if the target voice has no units at all.

The xinsert callbacks of the stream backend now handle the "removed"
notification callbacks properly.

Streaming xinsert clients now report buffer under/overflow. Disable
by passing the A2_RTSILENT flag to a2_OpenTap() or a2_OpenSend().

Closes #127.
Unified internal timestamp format

The A2S VM used to have a down-counting 24:8 timer for timing,
whereas messages and events use "absolute" timestamps related to
a state wide timebase. This made interactions between the
subsystems somewhat complicated and error prone. Now we're using
absolute timestamps throughout.

Closes #130.