Cache users

[froque]

Dbeaver uses multiple connections at the same time, leading to multiple requests to Vault and multiple temporary users created.

Additionally, this is a problem for read replica databases that may not process new users in time and reject the authentication

• https://developer.hashicorp.com/vault/docs/concepts/lease

[froque]

Vault response is

curl --header "X-Vault-Token: aaaaaaa" https://example.com/v1/database/creds/readonly

{
    "request_id": "43f5a069-a414-b4df-d49b-242161516dfe",
    "lease_id": "zzzzzzzz",
    "renewable": true,
    "lease_duration": 1209600,
    "data": {
        "password": "yyyyyy",
        "username": "xxxxxx"
    },
    "wrap_info": null,
    "warnings": null,
    "auth": null
}

lease_duration is in seconds

[froque]

fixed in #12