[Exim] [PATCH][CFT] Callout verification caching for Exim 4.…

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The attached patch (against Exim 4.10) implements a cache database for
Exim's callout verification. Each callout that results in either a
success or failure (but not deferrals for temporary errors) is entered
into the database with its success/failure result (and a timestamp,
which Exim does for all hint database records).

When a callout is requested, the cache is first checked to see if a
record for that address exists. If the record exists, and has not
expired (see below), the cached result is used and no callout is done.

If no record was found, the callout is done and then the result cached.

Record expiration is controlled by two new main configuration options:

callout_positive_expire (default 24 hours)
callout_negative_expire (default 2 hours)

These options take a single standard Exim time value.

The exim_dumpdb and exim_fixdb programs have also been updated to
support the callout cache database, so you can view its contents and
even remove/modify records if you so desire.

Please test this patch... I've got it running here and it's working
well, but more testing is always good. Philip, if there's anything you'd
like see done differently or expanded, let me know.
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[ callout-cache-patch of type application/x-java-applet deleted ]
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