Servers set change session on Substrate takes too long

[svyatonik]

Because of the pause between session restarts, connection issues and etc. it takes almost 2 minutes to complete migration from [\\Alice, \\Bob, \\Charlie] to [\\Alice, \\Bob, \\Charlie, \\Dave], even with empty databases.

Also - sometimes we run SSChange session several times (i.e. it is restarted). It is fine, but not in this case (small set changes, no keys at all).