Add AMM integration tests

[2opremio]

The tests should cover:

• The full lifecycle of a liquidity pool: creation, deposit, withdrawal, trading, removal and revocation
• All the new liquidity pool effects
• All the updated effects (due to the introduction of the liquidity pools)
• Fetching the new liquidity pool operations (deposit and withdraw)
• Querying all the new liquidity pool endpoints (e.g. liquidity pools, effects, by liquidity pool, operations by liquidity pool, transactions by liquidity pool).
• Path payments (send and receive) through a liquidity pool.

In order to run the tests we should update CircleCI to:

• Incorporate new jobs to run protocol 18 integration tests. You can see an example at [CAP 35] Separate CAP35 tests (so that we can merge cap35 to master) #3452 . Instead of using a protocol-specific name for the jobs we should probably use a more generic name (e.g. next_protocol) because we will be using separate integration tests for the future protocol releases.
• Those jobs Core with protocol 18 support (which requires a Core debian package and docker image built with --enable-next-protocol-version-unsafe-for-production ).

[bartekn]

Most cases handled in #3868. Going to add revoke case when stellar/stellar-core#3162 is merged.

[bartekn]

Image updated with new changes so this is no longer blocked.