lib/scanner: Speed up weak hash

The rolling version of adler32 is just a wrapper around the standard
hash/adler32 when used in a non-rolling fashion, but it's inefficient as
it allocates a new hash instance for every Write(). This uses the
default version instead in the block hasher, and adds a test to verify
the result is the same as they were before. It reduces allocations by
88% and increases speed about 5%.

	benchmark               old ns/op     new ns/op     delta
	BenchmarkHashFile-8     64434698      61303647      -4.86%

	benchmark               old MB/s     new MB/s     speedup
	BenchmarkHashFile-8     276.65       290.78       1.05x

	benchmark               old allocs     new allocs     delta
	BenchmarkHashFile-8     1238           150            -87.88%

	benchmark               old bytes     new bytes     delta
	BenchmarkHashFile-8     17877363      49292         -99.72%

lib/scanner/blocks.go

@@ -9,9 +9,9 @@ package scanner
 import (
 	"bytes"
 	"fmt"
+	"hash/adler32"
 	"io"
 
-	"github.com/chmduquesne/rollinghash/adler32"
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/sha256"
 )