Same contract can be added by differents users

[quentin-burg]

Current behavior:

• Contract address must be unique inside the Gas Station. So when a user add a contract, anybody cannot add the same contract.
• Users who want to add a contract that already exists have no information about this contract and in particular about the reason for refusal, which can be disturbing.

Behavior wanted

• Contracts may be added several times into Gas Station only if the owner is different (user A cannot add the same contract twice) => the couple (user ID * contract address) must be unique.
• Entrypoints can be enable or disabled depending on the user (=user A wants to enable entrypoint 1 while user B wants to enable entrypoint 2).
• Priority is given to the vault with the most credits.
• If a called entrypoint is deactivated, we will check whether an active entrypoint exists for the same contract for a different user. The credits needed to pay for the transaction will then be taken from the vault for that user.

Related to #21 (comment)