This is a cache of pairings of public keys and their corresponding message.
It accelerates aggregate verification when some public keys have already
been paired, and found in the cache.
We use it to cache pairings when validating transactions inserted into the
mempool, as many of those transactions are likely to show up in a full block
later. This makes it a lot cheaper to validate the full block.
However, validating a signature where we have no cached GT elements, the
aggregate_verify() primitive is faster. When long-syncing, that’s
preferable.