Add stream buffer flush threshold

[derekjn]

Let's go with a global (per-node) stream buffer size for now. Stream buffers should not be blindly flushed to disk by the background writer process. Instead, they should be flushed when evicted via standard LRU invalidation (when no other buffers are available to LRU flush).

Loop through all table buffers when evicting before looking at any stream buffers. When determining which buffer to flush to make room for a new one, stream buffers should look at evicting other stream buffers, and table buffers should look at other table buffers. If none can be found for a given buffer type, look at the other buffer type's buffers.

[derekjn]

Stream buffers should only be flushed under two conditions:

1. Stream buffer pool size exceeds its limit. In this case, we should flush a bunch of them to disk.
2. Process termination.