Add benchmarks for readVarInt.

This commit adds a few benchmarks for the readVarInt function.
Slightly optimize readVarInt.

This commit slightly optimizes the readVarInt function in the case of
multiple-byte variable length integers.  It also reduces the amount of
memory garbage it generates.

Before:

BenchmarkReadVarInt1     5000000               386 ns/op
BenchmarkReadVarInt3     5000000               693 ns/op
BenchmarkReadVarInt5     2000000               793 ns/op
BenchmarkReadVarInt9     5000000               709 ns/op

After:

BenchmarkReadVarInt1     5000000               387 ns/op
BenchmarkReadVarInt3     5000000               471 ns/op
BenchmarkReadVarInt5     5000000               575 ns/op
BenchmarkReadVarInt9     5000000               473 ns/op

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