reftable: convert from `strbuf` to `reftable_buf`

reftable: convert from `strbuf` to `reftable_buf`

Convert the reftable library to use the `reftable_buf` interface instead
of the `strbuf` interface. This is mostly a mechanical change via sed(1)
with some manual fixes where functions for `strbuf` and `reftable_buf`
differ. The converted code does not yet handle allocation failures. This
will be handled in subsequent commits.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Taylor Blau <me@ttaylorr.com>