Pjpersist

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- More precise flushing of datamanager to avoid unnecessary database
writes.

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- Enabled concurrent adds to IdNamesPJContainer by eliminating a query
that was causing transaction conflicts.

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- Python 3 compatibility fixes
- More efficient PJContainer.values() implementation

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- Need to protect all DB calls against `DatabaseDisconnected`

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- `psycopg2.OperationalError` and `psycopg2.InterfaceError` will be caught
on SQL command execution and reraised as `DatabaseDisconnected`

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- Added a new helper function to link subobject to main doc object. This is
needed when a custom `__getstate__()` and `__setstate__()` is implemented. A
detailed example is provided.

- Implemented `flush_hint` argument for `IDataManager.execute()` to allow
flushing only some objects during query. `flush_hints` is a list table names
that need to be flushed for the query to return a correct result.

- The Zope-specific containers use the `flush_hint` to only flush objects they
manage when a query is run on the container.

- While flushing objects, every main document object is now only flushed
once. Before that fix, any subobject would cause its doc object to be dumped
again.

Note: These optimizations provide a 15% performance improvements in real-world
applications.