common/options: Set osd_client_message_cap to 256.

common/options: Set osd_client_message_cap to 256.

This seems like a reasonable default value based on testing results here:
https://docs.google.com/spreadsheets/d/1dwKcxFKpAOWzDPekgojrJhfiCtPgiIf8CGGMG1rboRU/edit?usp=sharing

Eventually we may want to rethink how the throttles and even how flow control
works, but this at least gives us some basic limits now ( a little higher than
the old value of 100 that we used for many years).

Signed-off-by: Mark Nelson <mnelson@redhat.com>