Fix 64-bit alignment errors with request queue counters

This backports !5223 (merged) and !5226 (merged) to the 15-7-stable branch.

In 7c5bbf61, the addition of the ProtoFormat field to ObjectHash increased the number of bytes in the structure from 64 to 72. As a result, requestQueue now looks like:

type requestQueue struct {
	objectHash git.ObjectHash // 72 bytes
	outstandingRequests int64 // 8 bytes
	closed int32              // 4 bytes
	isReadingObject int32     // 4 bytes
	isObjectQueue bool        // 1 byte
	stdout *bufio.Reader      // 8 bytes
	stdin  *bufio.Writer      // 8 bytes
	trace *trace              // 8 bytes
}

Previously requestQueue totaled 112 bytes, which is 64-bit aligned. However, the addition of this new field in objectHash pushes the structure to 113 raw bytes. On a Raspberry Pi system, the Go compiler only guarantees 32-bit alignment, so we end up with a structure that is 116 bytes, which isn't 64-bit aligned.

Moving outstandingRequests to be first in the struct isn't enough. As https://github.com/golang/go/issues/11891#issue-97544684 says, in an array of structs, only the first element of a structure may be automatically 64-bit aligned. It depends if the size of each element, plus padding, is a multiple of 8.

To fix this mess, we now split these atomic counters in its own structure and add a test to ensure that it is 64-bit aligned.

Relates to #4701 (closed)

Changelog: fixed