Reduce the user space per-connection buffer size

The connection will only start executing new commands if the per-connection user space buffer is less than 40MB. There wasn't much work behind picking the limit (apart that it allows for retrieval of two documents of our max size).

On the bright side documents larger than 4k may be accounted for memory-wise in the resident size for the underlying engine (as we keep a reference to it in this case); unless we need to inflate the document for some reason (to strip off xattrs for instance). In that case we'll allocate additional memory for that.

We should probably reduce this user space buffer size limit to 1MB; which would allow ~256 operations on documents in the 4k range to be copied into the buffer.

With 1k of active clients filling their pipes we're up to 1GB in buffer space.