traffic_replay: Write group memberships once per group

traffic_replay: Write group memberships once per group

Each user-group membership was being written to the DB in a single
operation. With large numbers of users (e.g. 10,000 in average 15 groups
each), this becomes a lot of operations (e.g. 150,000). This patch
reworks the code so that we write the memberships for a group in
one operation. E.g. instead of 150,000 DB operations, we might make
1,500. This makes writing the group memberships several times
faster.

Note that rthere is a performance vs memory tradeoff. When we hit
10,000+ members in a group, memory-usage in the underlying DB modify
operation becomes very inefficient/costly. So we avoid potential memory
usage problems by writing no more than 1,000 users to a group at once.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>