fix build
Introduce a mta_limits structure to replace the hardcoded limits on the mta.
Various limits and delays are configurable on a per domain basis in smtpd.conf
using the following syntaxe:

limit for domain "my.domain" <keyword> <value> ...

The values are found in limit.c. They will be documented shortly.
They are still subject to changes.
Change the syntax to 'limit mta for domain "my.domain" ...' and
and introduce 'limit mta ...' to change the default settings.
Defaults are inherted by newly specified domains.
Fix the hang-on logic to work correctly with the transaction delay.
Count up to the limit instead of counting down to 0. The transaction
delay is taken into account in the keep-alive timeout.
do not accept a list in a virtual context
disable envelope cache for now
always return 0 if the envelope dump failed.
properly warn about failed envelope update.
typo
Simplify the imsg handling in queue_proc and queue_api.
warn and fatal on all imsg error.
helpers for creating response message
Reorganize code: put generic things at the beginning
Reorganize code as for queue_*.c
typo
Add proc backend for the scheduler API
plug scheduler_proc. things have to move from smtpd.h to smtpd-api.h