This exposes the ability to more efficiently create a block locator from
a chain view for a given block node by using their ability to do O(1)
lookups.
It also adds tests to ensure the behavior is correct.
This modifies the function to set the tip in the new chainview code to
bulk copy existing nodes when it needs to expand the cap rather than
simply creating a new empty slice and allowing the walk code below it to
repopulate it. This is a nice optimization since, in practice, most of
the time expanding the cap is only required when the active chain is
being extended after having run for a while which means the end result
is that it will be able to bulk copy all the nodes and just add the most
recent one versus having to walk them all and add them back.
Also, while here expand the tests for setting the tip to ensure the
nodes contained in the resulting view are correct after forcing the
resizes and correct a bug they exposed where changing between a
longer-shorter-longer chain where the longer chain is the same chain
could result in not populating the view correctly.
Finally, update the fake nodes generated by the tests to use a
nonce generated by a deterministic prng in order to ensure the hashes of
all fake nodes are unique, but reproducible.
This implements a new type in the blockchain package that takes
advantage of the fact that all block nodes are now in memory to provide
a flat view of a specific chain of blocks (a specific branch of the
overall block tree) from a given tip all the way back to the genesis
block along with several convenience functions such as efficiently
comparing two views, quickly finding the fork point (if any) between two
views, and O(1) lookup of the node at a specific height.
The view is not currently used, but the intent is that the code will be
refactored to make use of these views to simplify and optimize several
areas such as best chain selection and reorg logic and finding successor
nodes. They will also greatly simplify the process of disconnecting the
download logic from the connection logic.
A comprehensive suite of tests is provided to ensure the chain views
behave correctly.
