session_shared: negotiate init sz parameters MP-465

By default, session_shared will honor the paramters given by client.
An alternative, but not implemented, algorithm would be to only
re-use sessions with same sz parameters. This however, could reduce
re-use of sessions a lot. Hence this, simple algorithm instead. The
negotiation to a maximum sz values can be enabled by using
  <init maximum-record-size="sz1" preferred-message-size="sz2"/>
in configuration. This will make session_shared send those values
to backends and use returned init values in negotaition to
client before session_shared.