This commit is similar to 900d4908.

This fixes commit d90c5b83
("Allow NEO to store empty values").

This optimizes storage layout on disk, because more recent entries are often
being accessed more.

This will also simplify implementation of incremental backups.

Storage API is changed so that backends are not forced to use the checksum to
index data.

Values of '_oid_dict' attribute are never empty.

- Fix deadlock when client is in slave mode whereas a DB was open.
- Fix regression in commit b3b5175f
  ("tests: make it possible to run several threaded clusters at the same time").

No kidding !
And there was a test for this. Muhaha.

Storage.importFrom is changed for resumable imports.

Also, fix log message.

Fixes regression introduced in b6475dc3 .
So much for running test from neo folder...

Fixes a regression introduced in b5309c78 .

The only difference between an aborted connection and a closed one is that
packets sent just before aborting are pushed to the network before really
closing the connection. But further sends must fail, and nothing should be
received.

There is no need to react on every packet to mark a node as running. This
is updated on connection/disconnection, and it is sufficient.

Also, don't wait for client disconnection.
Also, factorise packet creation.

- Do not fetch data from outdated/discarded cells.
- Do not return more than transactions than requested by 'limit' parameter.
  Anyway, all results above this 'limit' could contain holes.