kvout gains append(), reserve(), adjust_length().

This is straccum-enough-like that msgpack will use it.
Also rename kvout::len to kvout::capacity.
Add Cmd_Replace.

Cmd_Replace is like replace: set the whole value, taking the
new value as a string. Also make run_replace like run_put
(it returns a result_t). Also rename logcmd_put1 to logcmd_replace
for consistency.