wire: Optimize message reads

[jrick]

This commit contains several performance and memory optimizations to improve message deserialization, both when generically called through the BtcDecode method of the Message interface and when reading wire protocol using ReadMessageN.

Special cases have been added for the bytes.Buffer and bytes.Reader reader types, avoiding the allocation and synchronization costs associated with using temporary buffers from the binary freelist and avoiding the heap allocations required due to the reader interface leaking the slice parameter.

ReadMessageN has reduced the number of allocations it must incur by avoiding the readElements helper function.

Finally, a new internal buffer type is introduced that is used exclusively by ReadMessageN. Since the lifetime and memory of this buffer is under the control of the wire package, and the buffer is never reset or truncated, this allows transaction script deserialization to slice the internal bytes of this buffer rather than pulling temporary buffers from the scriptPool freelist and copying these into a new contiguous allocation.

[jrick]

Using the newly added benchmark, before:

$ go test -run=^$ -bench=BenchmarkReadMessageN -benchmem 
goos: openbsd
goarch: amd64
pkg: github.com/decred/dcrd/wire
cpu: AMD Ryzen 7 5800X3D 8-Core Processor           
BenchmarkReadMessageN-8   	  334888	      3370 ns/op	     920 B/op	      15 allocs/op
PASS
ok  	github.com/decred/dcrd/wire	1.180s

After:

$ go test -run=^$ -bench=BenchmarkReadMessageN -benchmem 
goos: openbsd
goarch: amd64
pkg: github.com/decred/dcrd/wire
cpu: AMD Ryzen 7 5800X3D 8-Core Processor           
BenchmarkReadMessageN-8   	  537861	      1988 ns/op	    1432 B/op	       9 allocs/op
PASS
ok  	github.com/decred/dcrd/wire	1.106s

[davecgh]

Thanks for the PR. Nice changes overall. Saving several allocs on every single message read adds up to quite a few allocations over time, and will help to remove more GC pressure during the initial sync.