Knex cache for each tenant

[wallacetm]

Would be great and more performatic if we have a knex cache. This can be a new class or just a map, where the key can be the service_tenant and the value is the knex connection.

And we can create the Multischemase object like this:

Multischemase.Knex('service', 'tenant', ...configOptions);

Without a extra call of setContext('service', 'tenant').

With the knex cache we can solve the pool problem.