ZODB Components: Cosmetic: Use ImportLock() rather than aq_method_lock (a29456b).

It is exactly the same at the end but the name of the latter does not make
sense in this context. Moreover, it allows to get rid of Base import.
ZODB Components: Cosmetic: Remove useless imports.
ZODB Components: Do not mask exceptions when loading a Component.
ZODB Components: Do not store Persistent object in Component registry cache.
ZODB Components: Upon migration, Workflow History of ZODB Components must be kept.

Also, upon installation of bt5s, install WorkflowItem before Components as it
is needed to restore history.

Another solution would have been to validate() ZODB Components automatically
upon bt5 installation but it would mean losing 'modified' state information
and also imply that *all of them* will be validated even if a developer wants
to publish non-validated ZODB Components.
ZODB Components: Meaningless to remove ZODB Components migrated from FS.

This will not work when there are several nodes anyway, so just display a
message to the user instead.
Cosmetic: Since a29456b, Base.aq_method_lock is a context manager ('with').
ZODB Components: Fix bootstrap of migrated bt5s.

* Upon bt5 installation, install portal_type* items before bt5 {Document,
  Extensions...} as corresponding Portal Type are required once they have been
  migrated to ZODB Components.

* Likewise portal_{property_sheets, types}, portal_components must be created
  automatically *before* installing any bt5. This is required when Products
  will be migrated but also for bt5 items before bootstrapping erp5_core bt5.

* Set Permissions in ComponentTool instanciation and revoke all permissions,
  then allow only some of them for security sake.

* When creating ERP5 site with unit tests, add ERP5TypeTestCase to Developer
  Role ASAP so that there is no Permission issue when installing bootstrap bt5
  and test bt5s containing ZODB Components.