speed up agent-by-addr search

When this function was written, it was done under the assumption that
a) There are not a lot of agents, so linear search is fast enough
and
b) We just want it to work for now

However, it turns out that there can be a lot of agents, close to 1000
for the Qadim log for example. This means that there is quite some time
saving that we can do here, as get_agent_by_addr is used a lot in
set_agent_awares and set_agent_masters, so speeding this part up is
good!

We could build a HashMap, mapping the address to the agent (index), but
that would mean that we have to carry the hash map around. This patch
provides a simpler yet already good improvement: We invest a bit of time
after converting all agents to sort them by their address (as the agent
order is implementation defined anyway), so we can later use a binary
search to get the right agent. It's not O(1), as a hash map would be,
but it works in logarithmic time and already provides a big benefit:

Before
process Qadim           time:   [39.444 ms 39.501 ms 39.561 ms]

After
process Qadim           time:   [18.699 ms 18.744 ms 18.788 ms]
                        change: [-52.672% -52.546% -52.413%] (p = 0.00 < 0.05)

That is half of the processing time saved by a 3 line patch!

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