BUG/MEDIUM: stick: completely remove the unused flag from the store entries

The store[] array in the session holds a flag which probably aimed to
differenciate store entries learned from the request from those learned
from the response, and allowing responses to overwrite only the request
ones (eg: have a server set a response cookie which overwrites the request
one).

But this flag is set when a response data is stored, and is never cleared.
So in practice, haproxy always runs with this flag set, meaning that
responses prevent themselves from overriding the request data.

It is desirable anyway to keep the ability not to override data, because
the override is performed only based on the table and not on the key, so
that would mean that it would be impossible to retrieve two different
keys to store into a same table. For example, if a client sets a cookie
and a server another one, both need to be updated in the table in the
proper order. This is especially true when multiple keys may be tracked
on each side into the same table (eg: list of IP addresses in a header).

So the correct fix which also maintains the current behaviour consists in
simply removing this flag and never try to optimize for the overwrite case.

This fix also has the benefit of significantly reducing the session size,
by 64 bytes due to alignment issues caused by this flag!

The bug has been there forever (since 1.4-dev7), so a backport to 1.4
would be appropriate.
OPTIM: ebtree: pack the struct eb_node to avoid holes on 64-bit

struct eb_node is 36 bytes on a 64-bit machine. It's thus rounded
up to 40 bytes, and when forming a struct eb32_node, another 4 bytes
are added, rounded up to 48 bytes. We waste 8 bytes of space on 48
bytes because of alignments. It's basically the same with memory
blocks and immediate strings.

By packing the structure, eb32_node is down to 40 bytes. This saves
16 bytes per struct task and 20 bytes per struct stksess, used to
store each stick-table key.