[server] Make connection pooler configurable and set better default values
Drop the configuration connections-pool-size and add new configurations options:
* connections-pool-min-size. Set to 0 by default so we open connections only
when needed. This avoid opening min-size*processes connections at startup,
which is, it think, a good default.
* connections-pool-max-size. Set to 0 (unlimited) by default, so we move the
bottleneck to postgresql.
* connections-idle-timeout. Set to 10 minutes. I don't have arguments about
this except that this is the default in pgbouncer.
Metadata
--------
.. index::
schema: meta-data;
schema: eid; creation_date; modification_data; cwuri
schema: created_by; owned_by; is; is_instance;
Each entity type in |cubicweb| has at least the following meta-data attributes and relations:
`eid`
entity's identifier which is unique in an instance. We usually call this identifier `eid` for historical reason.
`creation_date`
Date and time of the creation of the entity.
`modification_date`
Date and time of the latest modification of an entity.
`cwuri`
Reference URL of the entity, which is not expected to change.
`created_by`
Relation to the :ref:`users <CWUser>` who has created the entity
`owned_by`
Relation to :ref:`users <CWUser>` whom the entity belongs; usually the creator but not
necessary, and it could have multiple owners notably for permission control
`is`
Relation to the :ref:`entity type <CWEType>` of which type the entity is.
`is_instance`
Relation to the :ref:`entity types <CWEType>` of which type the
entity is an instance of.