ack_tracker: keep AckTracker parent class 64 bit aligned

AckTracker is a parent "class" for LateAckTracker which contains a RingBuffer.
RingBuffer has a pointer member which points to the memory where the AckRecord
slots (containing Bookmark) are stored.
When the AckTracker is unaligned then the RingBuffer within LateAckTracker
will also be unaligned and that can lead to SIGBUS on Sparc and IA64 platforms.

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>
Merge pull request #169 from lbudai/f/merge-proto

ack_tracker: keep AckTracker parent class 64 bit aligned
bookmark: eliminate paddings (while keeping alignment well)

Using those paddings was a mistake: it makes code too breakable.
If we want to avoid bus error(or any other alignment issues)
on some stricter architectures(eg.:Sparc, IA64) what we have to do is
keep Bookmark aligned. For that reason we should choose carefully the
internal representation of the Bookmark.

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>
Merge pull request #170 from lbudai/f/merge-proto

bookmark: eliminate paddings (while keeping alignment well)