buffer: always start off with BUFFER_INIT_SIZE

Initialize all new buffers with a BUFFER_INIT_SIZE allocation right
off the bat, to avoid later byte-by-byte repeated realloc calls.
buffer: never realloc down in buffer_reinit

Never realloc buffers down to 127 bytes in buffer_reinit: during large
transmits, this causes a "realloc ping-pong" with the session out buffer
constantly being realloc'd down to 127 bytes, just to be realloc'd back
up to 32 kilobytes for the next packet send (for example).

Were the realloc behavior desired in buffer_reinit, it would be better
to issue a free followed by malloc (as-is, 127 bytes that we don't care
about are memmove'd to the new buffer).
packet: remove extra memset of 'padstring'
packet: elide two buffer_prepend calls into one

In packet_send2, rather that issue two separate buffer_prepend_data calls
(each of which may entail realloc + memmove + memcpy), elide the prepend
work into a single buffer_prepend_data: the header information is computed
locally, and a single 5 byte prepend operation is now done instead of
prepending 1, then 4 bytes.
buffer: reserve PACKET_SEND_HEADER_SIZE for packet_send2