Failure of db+write transaction when io error

Basic implementation.

Simple repository ok.
With too deep repository: RuntimeError maximum recursion depth exceeded

This reverts commit 675a52e2.

This is indeed `provably` if the code is implemented correctly
which it is :D (or will be soon)

```sh
$ make coverage
nosetests3 --with-coverage swh -v --cover-package=swh
check_folder_path_computation (test_file.TestFile) ... ok
check_folder_path_computation_with_different_deps (test_file.TestFile)
... ok
check_write_data_with_compress_flag_on (test_file.TestFile) ... ok
check_write_data_with_no_compress_flag (test_file.TestFile) ... ok
compute_blob_sha1 (test_hash.TestHash) ... ok
compute_blob_sha1_2 (test_hash.TestHash) ... ok
compute_sha1_data (test_hash.TestHash) ... ok
compute_sha1_data2 (test_hash.TestHash) ... ok
Trigger loader and make sure everything is ok. ... ok

Name                    Stmts   Miss  Cover   Missing
-----------------------------------------------------
swh                         0      0   100%
swh.db                     18      0   100%
swh.file                    8      0   100%
swh.gitloader               0      0   100%
swh.gitloader.loader       63      7    89%   48-50, 53-55, 65-66, 135
swh.gitloader.models       25      0   100%
swh.gitloader.storage      18      0   100%
swh.hash                   13      0   100%
-----------------------------------------------------
TOTAL                     145      7    95%
----------------------------------------------------------------------
Ran 9 tests in 0.483s

OK
```

psycopg2 already permits to use cursor connection with `with`

Activating the debug mode (flag `-v` or `--verbose` will log more information in the following format:
<action-verb> <nature-object> <sha1-name-or-path>

where:
<action-verb>
- walk       walk a tree or a reference
- skip       skip an already saved/visited object or unknown object (e.g. commit submodule)
- store      save an object in db (file or object) and content (file or object) storage
- initialize Initialize the db
- clean      Clean the db's data

<nature-object>
- tree
- commit
- blob
- reference
- submodule-commit A commit from a submodule
- unknown-action   An unknown action from swhgitloader's cli

<sha1-name-or-path>
- sha1 git or swh's sha1
- name object name
- path object's content storage path

Activating the debug mode (flag `-v` or `--verbose` will log more information in the following format:
<action-verb> <nature-object> <sha1-name-or-path>

where:
<action-verb>
- walk       walk a tree
- skip       skip an already saved/visited object or unknown object (e.g. commit submodule)
- store      save an object in db (file or object) and content (file or object) storage
- inject     serialize on disk an object
- initialize Initialize the db
- clean      Clean the db's data

<nature-object>
- tree
- commit
- blob
- reference
- submodule-commit A commit from a submodule

<sha1-name-or-path>
- sha1 git or swh's sha1
- name object name
- path object's content storage path