[Resolves #26] Support configuration for skip checking of schema existence before switching

[rpbaltazar]

This is configurable by setting: tenant_presence_check. It defaults to true
in order to maintain the original gem behavior. This is only checked when using one of the PostgreSQL adapters.
The original gem behavior, when running switch would look for the existence of the schema before switching. This adds an extra query on every context switch. While in the default simple scenarios this is a valid check, in high volume platforms this adds some unnecessary overhead which can be detected in some other ways on the application level.

Setting this configuration value to false will disable the schema presence check before trying to switch the context.

Apartment.configure do |config|
  tenant_presence_check = false
end