Reorder Scanner fields to fix alignment issues

[WGH-]

Go doesn't always guarantee 64-bit alignment of 64-bit integer, which leads to crashes when atomic operations are used on them on certain platforms (e.g. ARM).

This problem is described here: https://golang.org/pkg/sync/atomic/#pkg-note-BUG

By putting these fields first, proper alignment will be guaranteed.

golang/go#599
golang/go#23345

This is how crash looks like:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x4 pc=0x11f70]

goroutine 37 [running]:
github.com/google/certificate-transparency-go/scanner.(*Scanner).processEntry(0xfb2120, 0x1fb09886, 0x0, 0xf06000, 0x62e, 0x630, 0x10de000, 0xbda, 0xbdc, 0x3570f0, ...)
        /home/wgh/go/src/github.com/google/certificate-transparency-go/scanner/scanner.go:108 +0x34
github.com/google/certificate-transparency-go/scanner.(*Scanner).matcherJob(0xfb2120, 0xd4e2c0, 0x3570f0, 0x3570f0)
        /home/wgh/go/src/github.com/google/certificate-transparency-go/scanner/scanner.go:183 +0x80
github.com/google/certificate-transparency-go/scanner.(*Scanner).ScanLog.func2(0xc105e0, 0xfb2120, 0xd4e2c0, 0x3570f0, 0x3570f0, 0x1)
        /home/wgh/go/src/github.com/google/certificate-transparency-go/scanner/scanner.go:287 +0xd0
created by github.com/google/certificate-transparency-go/scanner.(*Scanner).ScanLog
        /home/wgh/go/src/github.com/google/certificate-transparency-go/scanner/scanner.go:284 +0x200
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x4 pc=0x11f70]

[codecov]

Codecov Report

Merging #464 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #464   +/-   ##
=======================================
  Coverage   66.55%   66.55%           
=======================================
  Files          88       88           
  Lines       10658    10658           
=======================================
  Hits         7093     7093           
  Misses       2964     2964           
  Partials      601      601

Impacted Files	Coverage Δ
scanner/scanner.go	56.68% <ø> (ø)	⬆️

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[daviddrysdale]

Well spotted, thank you.