ds: simplify write buffer accounting

ds: simplify write buffer accounting

Keeping track of write_buf_size was redundant and pointless when
we can simply check the number of elements in the buffer array.
Multiple sources of truth leads to confusion; confusion leads to
bugs.

Finally, rename the prefixes to 'wbuf' to ensure we loudly
(instead of silently) break any external dependencies being
ported over from Danga::Socket, as further changes are pending.