to test async operation of the syncing core while using the synchronous
maildir driver, we add a mode to the proxy driver where it queues
callback invocations to the next main loop iteration.
use the indentation of the placeholder, not the replacement.
this doesn't matter right now, as all placeholders are indented by one
step, but that will change soon.
the indent function cannot be inlined into the substitution, as for some
reason ^ then matches the end of the string, not the embedded line
starts (with perl v5.32). also, $1 needs to go into a temporary anyway.
do that by wrapping the actual stores into proxies.
the proxy driver's code is auto-generated from function templates, some
parameters, and the declarations of the driver functions themselves.
attempts to do it with CPP macros turned out to be a nightmare.
