In cases where many small writes are made to a stream
lacking _writev, the array data structure backing the
WriteReq buffer would greatly increase GC pressure.

Specifically, in the fs.WriteStream case, the
clearBuffer routine would only clear a single WriteReq
from the buffer before exiting, but would cause the
entire backing array to be GC'd. Switching to [].shift
lessened pressure, but still the bulk of the time was
spent in memcpy.

This replaces that structure with a linked list-backed
queue so that adding and removing from the queue is O(1).
In the _writev case, collecting the buffer requires an
O(N) loop over the buffer, but that was already being
performed to collect callbacks, so slowdown should be
neglible.