This mimics what FileStorage uses (file offsets) but in a relational manner.
This offloads decision of the ability to undo a transaction to storages,
avoiding 3 data loads for each object in the transaction at client side.
This also makes Neo refuse to undo transactions where object data would happen
to be equal between current value and undone value.
Finally, this is required to make database pack work properly (namely, it
prevents loosing objects which are orphans at pack TID, but are reachable
after it thanks to a transactional undo).

Also, extend storage's transaction manager so database adapter can fetch data
already sent by client in the same transaction, so it can undo multiple
transactions at once. Requires making object lock re-entrant (done in this
commit).

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OID 0 is a very valid oid. It's often used for the most important object of
the database: the root object. Using 0xffffffffffffffff leaves some space.

TID 0 is a very valid tid, for transactions back in unix year 0... or
ZODB tests. Using 0xffffffffffffffff leaves some space.

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Test case (added):
- Start a cluster with two storages
- Restart the whole cluster except one storage
- Allow recover from existing storage
- The admin crash when getting the new partition table because it doesn't
known the second (down) node.

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BLOB is 2^16 bytes
MEDIUMBLOB is 2^24 bytes
LONGBLOB is 2^32 bytes

Choose LONGBLOB to store bigger objects.

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ReadBuffer join received strings only when all requested data is available.
This avoid many useless data copies in case of big packets. The gain factor
is about 50x for a 25MB packet.

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We currently support only python 2.4 for client (zope) side.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@1950 71dcc9de-d417-0410-9af5-da40c76e7ee4

The logger use dispatch tables from EventHandler to known the expected
method name then retreive its own packet handler.

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It's just broken to do this way (I intended to locally revert that commit,
instead of pushing it to the repository... Sorry).

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