[MEDIUM] improve behaviour with large number of servers per proxy

[MEDIUM] improve behaviour with large number of servers per proxy

When a very large number of servers is configured (thousands),
shutting down many of them at once could lead to large number
of calls to recalc_server_map() which already takes some time.
This would result in an O(N^3) computation time, leading to
noticeable pauses on slow embedded CPUs on test platforms.

Instead, mark the map as dirty and recalc it only when needed.