Previously there was only a function to get the latest checkpoint. This
commit exposes a new function named Checkpoints which returns a slice of
checkpoints ordered by height for the active network or nil when
checkpoints are disabled.
Previously the code performed a database query for every checkpoint (going
backwards) to find the latest known checkpoint on every block. This was
particularly noticabled near the beginning of the block chain when there
are still several checkpoints that haven't been reached yet.
This commit changes the logic to cache the latest known checkpoint while
keeping track of when it needs to be updated once a new later known
checkpoint has been reached.
While here, also add a log message when a checkpoint has been reached and
verified.
Previously the main network checkpoints were being used for unrecognized
networks. This commit changes the code so that no checkpoints are used in
that scenario.
After discussing the criteria used by the core developers on #btc-dev IRC
channel, gmaxwell indicated they like to see at least 2016 blocks. This
commit updates the checkpoint confirmations accordingly.
