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This branch uses a single connection for catalog and start a transaction with REPEATABLE-READ or READ-COMMITTED isolation level, based on the first query. If the first query contains SELECT, REPEATABLE-READ is used. Otherwise READ-COMMITTED is used. For activity tables, READ-COMMITTED is always used.

Some exceptions:

• z_get_table_schema: it can be called outside MariaDB transaction and is not couted as 'first SELECT' query.
• z_getitem_by_path and z_getitem_by_uid: they are used only at the beginning of indexation, thus we force start READ-COMMITTED transaction.

With this branch, the code compatibility should be mostly kept.

10.4
  duration: 11077.6
  deadlock: 5329

10.11
  duration: 20205.9
  deadlock: 129649

11.4
  duration: 18263.4
  deadlock: 72108

11.4-bis
  duration: 9774.21
  deadlock: 16775

11.4-unsafe
  duration: 9354.08
  deadlock: 6055

/cc @vpelletier, @jerome