ObjectIdSerializer: Support serialization of known non-null ObjectId

The implementation of ObjectIdSerializer, added in change I7599cf8bd,
is not equivalent to the original implementation in Gerrit [1].

The Gerrit implementation provides separate methods to (de)serialize
instances of ObjectId that are known to be non-null. In these methods,
no "marker" is written to the stream. Replacing Gerrit's implementation
with ObjectIdSerializer [2] broke persistent caches because it started
writing markers where they were not expected [3].

Since ObjectIdSerializer is included in JGit 4.11 we can't change the
existing #write and #read methods. Keep those as-is, but extend the
Javadoc to clarify that they support possibly null ObjectId instances.

Add new methods #writeWithoutMarker and #readWithoutMarker to support
the cases where the ObjectId is known to be non-null and the marker
should not be written to the serialization stream.

Also:

- Replace the hard-coded `0` and `1` markers with constants that can
  be linked from the Javadocs.

- Include the marker value in the "Invalid flag before ObjectId"
  exception message.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/9792
[2] https://gerrit-review.googlesource.com/c/gerrit/+/165851
[3] https://gerrit-review.googlesource.com/c/gerrit/+/165952

Change-Id: Iaf84c3ec32ecf83efffb306fdb4940cc85740f3f
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>