[Server v3] Define, implement and test causality conditions systematically

[vxgmichel]

The v2 parsec server used to explicitly list and implement causality conditions that have to be met in order to accept a write requests.

Example with api_realm_update_roles:

parsec-cloud/parsec/backend/realm.py

Lines 351 to 376 in d397c03

	async def api_realm_update_roles(
	self, client_ctx: AuthenticatedClientContext, req: RealmUpdateRolesReq
	) -> RealmUpdateRolesRep:
	"""
	This API call, when successful, performs the writing of a new role certificate to the database.
	Before adding new entries, extra care should be taken in order to guarantee the consistency in
	the ordering of the different timestamps stored in the database.
	In particular, the backend server performs the following checks:
	- The certificate must have a timestamp strictly greater than the last certificate for
	the same user in the same realm.
	- If the certificate corresponds to a role without write rights, its timestamp should
	be strictly greater than the timestamp of the last vlob update performed by the
	corresponding user in the corresponding realm.
	- If the certificate corresponds to a role without management rights, its timestamp should
	be strictly greater than the timestamp of the last role certificate uploaded by the
	corresponding user in the corresponding realm.
	- If the certificate revokes some owner rights, its timestamp should be strictly greater than
	the timestamp of the last archiving certificate uploaded by the corresponding user in the
	corresponding realm.
	If one of those constraints is not satisfied, an error is returned with the status
	`require_greater_timestamp` indicating to the client that it should craft a new certificate
	with a timestamp strictly greater than the timestamp provided with the error.
	The `api_vlob_create` and `api_vlob_update` calls also perform similar checks.

The v3 server changed those conditions but they are not implemented and tested systematically.

They should be documented, and implemented the same way between the memory and PostgreSQL implementation.
They should also be tested systematically.

For instance, here's a difference between the memory and postgre implementation:

parsec-cloud/server/tests/api_v4/authenticated/test_realm_create.py

Lines 165 to 172 in f1ccaa5

	# The rotation of a realm has nothing to do with the creation of a new realm
	# The postgre implementation does not perform this check
	if with_postgresql:
	assert rep == authenticated_cmds.v4.realm_create.RepOk()
	else:
	assert rep == authenticated_cmds.v4.realm_create.RepRequireGreaterTimestamp(
	strictly_greater_than=last_certificate_timestamp
	)