PID lookup, streaming pad filter and sbuf alloc optimizations

There is high probability (because of the "nature" of TS streams) that
next PID will match previous one. This patch tries to do simple "caching"
of last PID to speedup PID lookups.

The streaming pad filter is handled faster way now.

Also, sbuf allocations routines are optimized (better sb_size prediction and
allocation routine is called only on demand - not all time).