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>

src/common/options/osd.yaml.in

@@ -629,7 +629,7 @@ options:
   type: uint
   level: advanced
   desc: maximum number of in-flight client requests
-  default: 0
+  default: 256
   with_legacy: true
 - name: osd_crush_update_on_start
   type: bool