Some initial buffer sizes are pretty large

Once we have generic retry-resize support across the code in this library we should audit some of the initial sizes such that they make sense as _initial_ sizes.

While working on the RFC for retry support @nixpanic noted the size for the config option buffer was rather large:

Quoth:

This buffer looks pretty large for a configuration option value. Of course this was the earlier default, but starting with 128 bytes would probably be sufficient for many options (booleans, integers, names, ...).

_Originally posted by @nixpanic in https://github.com/ceph/go-ceph/pull/191_