Don't import psycopg2.tz into the C extension

This makes possible to import _psycopg directly, after adding the
package directory to the pythonpath. This enables hacks such as:

    sys.path.insert(0, '/path/to/psycopg2')
    import _psycopg
    sys.modules['psycopg2._psycopg'] = _psycopg
    sys.path.pop(0)

which can work around e.g. the problem of #201, freeze that cannot
freeze psycopg2. Well, freeze cannot freeze it because it's just not
designed to deal with C extensions. At least now the frozen application
can hack the pythonpath and work around the limitation by importing
_psycopg as above and then doing the rest of the imports normally.

Keeping long-lived references to python objects is bad anyway: the
tz module couldn't be reloaded before.
Added test to verify _psycopg can be imported
Fixed memory leak with large objects

Deallocating closed large objects failed to decrement the connection
refcount. The fact the lobject is closed doesn't matter for refcount.
Issue detected by the always useful scripts/refcounter.py

With an extra bit of unrequested whitespace love.
Clear adapters in test to maintain reference count
Dropped creation of errcodes with missing spec field

On further inspection these names are just aliases for values already
defined: we don't need the duplication.