Deprecate pool support in favor of generic-pool

[kadler]

Based on discussion stemming from #63 (comment)

This package gives us a lot of benefits:

• Supports what our built-in pool code does and more
• Allow us to close a bunch of issues we have with the current pool implementation
• Allow us to drop the async package

In addition, it supports a user-defined validation function, which allows the user to customize how they want to determine if a connection is valid. Currently we only check the "connected" attribute, which queries SQL_ATTR_CONNECTION_DEAD that only gives the last known status of the connection. If the connection has died since it was last used, this check will do nothing.

Goals of this would be:

• Mark the Pool class as deprecated and to be removed in the future (v3? v2.4 + XX?)
• Document basic usage of generic-pool with node-odbc or provide an example

https://www.npmjs.com/package/generic-pool