Overall, we can use LANGUAGE sql only in 6 cases (vs 17 functions that
remains plpgsql). This is in large part due to the fact that LANGUAGE
sql stored procedures cannot non-existent tables, and most notably
temporary tables.

Closes T55.

the functions no longer fail with internal errors (due to "insert into
... strict"), but rather return NULL in those cases, because no match
has been found.

include upgrade script from 16

The function is parametric in the type of entries to be
added (file/dir/rev).

Get rid of the previous 3 (99% overlapping) functions
swh_directory_{file,rev,dir}_entry_add functions. Make storage.py use
the new function.

The triplet of None should be sandboxed to db.py

Remove the needless yield

by uniting on contents that appear to be missing when looking at a
single checksum algorithm. This is semantically equivalent to the
previous approach, but allows to exploit single-column indexes; which
are non-redundant, smaller, and hence more likely to be used.

This is the "metro ligne 6" approach.

This currently uses pickle which breaks the msgpack + dict "abstraction"
we settled on. This should be fixed eventually.

with unified api

Closes T44.

This removes tables directory_list_{file,dir,rev}, and add corresponding
arrays {dir,file,rev}_entries to the directory table, which was just a
set up to now.

Indexes migrated accordingly.