Support Nim 2.x, compatibility with waterpark.

- Nim 2.x has moved the DB connectors outside the standard library to
  the `db_connector` package.
- Refactor the pooling implementation and macro expectations to use the
  `withConnection` name instead of `withConn`. This change allows a
  caller to use a [waterpark](https://github.com/guzba/waterpark) pool
  instance instead of the builtin pool instance. Waterpark provides
  better support for multi-threaded environments. The builtin pooling
  mechanism may be deprecated in favor of waterpark in the future.
- Add the `getModelIfItExists` generated proc to the list of standard
  procs we generate. This is a flavour of `getModel` that returns an
  `Option` instead of raising an exception when there is no model for
  the given id.
- Change `PaginationParams#orderBy` to accept a `seq[string]` to allow
  for sorting on multiple fields.

src/fiber_orm/db_common.nim

@@ -1,3 +1,3 @@
-import std/[db_postgres, db_sqlite]
+import db_connector/[db_postgres, db_sqlite]
 
 type DbConnType* = db_postgres.DbConn or db_sqlite.DbConn