The caveat is that it will only be supported when we will be using
python3 >= 3.5.

Related T692

Related T700

Closes T255
Ref. D179

Found by investigating T75

The subversion loader (T680) has shown that throwing floating point values
around for timestamps is a mess waiting to happen.

We now coerce all clients to send us timestamps as integer numbers of seconds
and microseconds, avoiding data losses everywhere.

- default
- ignoring empty folders
- ignore folder based on pattern in names

Combination of:
- validation on files
- ignore empty folder

Since we compute it anyway, better return it along with the result

Decrease the number of paths to compute to 1 common ancestor (if any):
- Scan only that directory and rehash with new results (data changed)
- Update the resulting objects with those new hashes.
- Update from that directory to the rootdir the existing hashes computation

level for example)

Move timestamp normalization to another function to make it more easily
movable.

This function wasn't in sync with what's supposed to be our revision
schema

so no bytes, and we enforce during the checksum computation function

T270