Update jitter buffer to avoid/reduce such case of shrinking and underflow occuring one after another.

[nanang]

Proposed solutions:

1. Shrinking to be based on recent maximum burst level. This will make jitter buffer shrinking a bit less aggressive. Currently shrinking is done based on current burst level (presented by jb_prefetch).
2. Reenable prefetch buffering. This should be able to be configured in run-time, e.g: setting initial-prefetch > 0 will make this feature enabled.

[nanang]

In r2310:

• Jitter buffer shrinking is done based on max_hist_level (recent maximum burst level).
• Updated jitter buffer to maintain max_hist_level instead of resetting it each time current level updated.
• Updated default jitter buffer size in stream.c from 360ms to 500ms to accomodate more burst level.

[nanang]

In r2317: Reenable prefetch buffering via initial prefetch setting (set 0 to disable, otherwise to enable).

[nanang]

r2318: Cleaned up compilation warning in jbuf.

[nanang]

r2321: Fixed assertion caused by prefetch > min_prefetch (thanks Michael Bradley Jr for the report).