Review bufferPool usages for concurrent slots

[jfallows]

WriteStream.getWriteBuffer(...) can use BufferPool.byteBuffer(...) on an existing slot in some cases.

WriteStream.handleUnwrittenData(...) can acquire a new slot to store remaining data and once again accesses the BufferPool.byteBuffer for the new slot.

There is potential for offset and limit of the original byte buffer to be reset unintentionally.

[cmebarrow]

It's not actually possible for getWriteBuffer and handleUnwrittenData to manipulate different slots because the logic in each is guarded by the if (slot == NO_SLOT) condition. handleUnwrittenData will only acquire a new buffer if slot == NO_SLOT, in other words, if the prior call to getWriteBuffer did not acquire a slot.

[jfallows]

Prefer we interact with BufferPool to access the byteBuffer on stack whenever possible as it makes the code easier to reason about and also more easily prevents undesirable side effects.