This removes the root field and all references to it from the BlockChain
since it is no longer required.

It was previously required because the chain state was not initialized
when the instance was created.  However, that is no longer the case, so
there is no reason to keep it around any longer.

This adds a benchmark for deserializing a large transaction that is
often referred to as the megatransaction since it is the largest Bitcoin
transaction mined to date.  It consists of 5569 inputs and 1 output and
its hash is:

bb41a757f405890fb0f5856228e23b715702d714d59bf2b1feb70d8b2b4e3e08.

This is being done so there is a benchmark that tests more of a
worst-case scenario which is a better candidate for identifying and
testing improvements.

The following benchmark results shows the how much more intensive this
transaction is over the existing mock transaction:

DeserializeTxSmall  1000000    1751 ns/op      376 B/op     16 allocs/op
DeserializeTxLarge  300     5093980 ns/op  1672829 B/op  33430 allocs/op

Merges in btcd commit de4fb24.

Merges in btcd commit 27c0f9f.