In the inbox loop, before getting the userdata, flush the object cache if
the default object cache is in use.

This is done because the default object cache is only meant to be used on a
single page load.  Since RBE runs in the background, we need to flush the
object cache so WP will do a direct DB query the next time it fetches any
data.

To check if the default object cache is in use, this commit introduces the
bp_rbe_is_external_object_cache_used() function.

See #28.
Revert "First attempts at clearing user object cache after failure. See #28."

This reverts commit 0149314d540267dfccf811f9eda4e2972dbfbf2a.
In the inbox loop, instead of using email_exists() use get_user_by().

This is so we don't have to re-query the userdata later on when checking if
the user is a spammer.  A performance enhancement!
Boo Ray.
Tweak email signature removal.
Clear listener object after modifying mail headers.

This is to prevent RBE from modifying any non-RBE emails unintentionally.
For WP Better Emails support, do a better job removing the existing RBE
marker from the email.
For WP Better Emails support, check for non-RBE emails by looking for the
non-RBE marker and bailing if it's found.

See commit bbf79e0.