Optimize transaction Deserialize.

The benchmark results for the current commit:

Before: BenchmarkDeserializeTx    500000              4018 ns/op
After:  BenchmarkDeserializeTx    500000              3780 ns/op

The cumulative benchmark results since commit b7b700fd5aebb86addc93efecaa367a2f4976058:

Before: BenchmarkDeserializeTx    200000             10665 ns/op
After:  BenchmarkDeserializeTx    500000              3780 ns/op

This is part of the ongoing effort to optimize serialization as noted in
conformal/btcd#27.