This commit corrects the reading of the serialized height in coinbase
transactions for block height of version 2 or greater. On mainnet, the
serialized height is always 3 bytes and will continue to be so for
something like another ~159 years, so there was no issue with mainnet.
However on testnet, there are some version 2 blocks which are low enough
in the chain to only take 2 bytes to serialize.
In addition, this commit adds a full tests for the relavant function
including negative tests and variable length serialized lengths for block
heights.
Closes#1.
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.
