Let client response windows override init_delay

Allow client's response_window lower than main_config.init_delay to
override it. This allows initial request processing delay to track
client response windows, in addition to home server response windows.
Add max_response_timeouts option

Add "max_response_timeouts" - a home server option specifying number of
times replies are allowed to miss "response_window" before the server
enters the zombie period.

This allows more tolerance before transiting to zombie period for lower
response window configurations.
Make the configuration more understandable.  Fixes #678

It should just be "response_timeouts", and not "max_response_timeouts"
it's not an upper limit, and there's no "min" limit.

Setting "ignore this many replies missing" is hard to understand.
It's easier to say "trigger zombie period after response_timeouts
packets are lost".
Add sql_unset_user().  Fixes #640

It's really just a macro around pairdelete.  But it's clearer
to have sql_set_user() / sql_unset_user, than to use pairdelete
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