This commit modifies the double spend detection to handle double spends
within the same block as well as side chains when doing the checks before
reorganizing the chain.
In addition to returning errors to the caller, log the error with a prefix
in a few key places that helps identify the origin for errors. In some
cases, the underlying error comes from a different subsystem such as the
SQL database driver and the error messages can be fairly generic.
This commit modifies the code to use params based on the active network
which paves the way for supporting the special rules and different genesis
blocks used by the test networks.
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.
