Driver improvements for Dolt

[firelizzard18]

It turns out Dolt needs to have control over how sql.Sessions are created, and it needs to modify the *sql.Context prior to query execution. This MR adds two interfaces:

• driver.SessionBuilder
• driver.ContextBuilder

When a driver is created, with driver.New, if the provider implements either of these interfaces, then the provider's implementation will be used for creating sessions/contexts. Otherwise, a default implementation is used. The Dolt provider can then implement those two interfaces and do the necessary work to setup the session/context:

• New SQL session
  • Create a dolt session (wrapping a sql session)
  • For each Dolt database in the SQL catalog:
    • Call doltSession.AddDB
• New SQL context
  • Create a sql context
  • For each Dolt database in the SQL catalog:
    • Get the working root of the corresponding Dolt env
    • Call doltDatabase.SetRoot
    • Call sqle.RegisterSchemaFragments

Prior to adding SessionBuilder, attempting to execute a query would panic, when Dolt attempts to coerce the session into a Dolt session. Prior to adding ContextBuilder, some features would not work - for example, queries had no access to uncommitted changes.

For reference, this is the Dolt driver implementation I'm working on.

[zachmu]

LG, thanks for the contribution!