db: add shadow transactions

Creating a transaction can be very time consuming on large filters since we
create a duplicate filter tree iteratively using the rules supplied by the
caller.  In an effort to speed this up we introduce the idea of shadow
transactions where on a successful transaction commit we preserve the old
transaction checkpoint and bring it up to date with the current filter and
save it for future use.  The next time we start a new transaction we check
to see if a shadow transaction exists, if it does we use that instead of
creating a new transaction checkpoint from scratch.

Acked-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

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