Preallocate memory for the message header in WriteMessageN and the
serialization buffer in MsgTx's TxSha().

Benchmarking shows this is slightly faster due to avoiding the extra
garbage collection in addition to less peak memory usage.

Before: BenchmarkTxShaOld  500000         5626 ns/op
After:  BenchmarkTxShaNew  500000         5457 ns/op