Migrate to simpler/decoupled factory in driver

This allows more freedom on how the connection is created. It will no longer need to have an explicit reference to the connection URI

spec/custom_drivers_types_spec.cr

@@ -47,8 +47,8 @@ class FooDriver < DB::Driver
     @@row
   end
 
-  def build_connection(context : DB::ConnectionContext) : DB::Connection
-    FooConnection.new
+  def connection_builder(uri : URI) : Proc(DB::Connection)
+    -> { FooConnection.new.as(DB::Connection) }
   end
 
   class FooConnection < DB::Connection
@@ -110,8 +110,8 @@ class BarDriver < DB::Driver
     @@row
   end
 
-  def build_connection(context : DB::ConnectionContext) : DB::Connection
-    BarConnection.new
+  def connection_builder(uri : URI) : Proc(DB::Connection)
+    -> { BarConnection.new.as(DB::Connection) }
   end
 
   class BarConnection < DB::Connection

spec/dummy_driver.cr

@@ -2,8 +2,8 @@ require "spec"
 require "../src/db"
 
 class DummyDriver < DB::Driver
-  def build_connection(context : DB::ConnectionContext) : DB::Connection
-    DummyConnection.new
+  def connection_builder(uri : URI) : Proc(DB::Connection)
+    -> { DummyConnection.new.as(DB::Connection) }
   end
 
   class DummyConnection < DB::Connection

src/db.cr

@@ -161,7 +161,7 @@ module DB
 
   private def self.build_connection(uri : URI)
     # PENDING: parse connection options from uri and set the right connection context
-    build_driver(uri).build_connection(SingleConnectionContext.default).as(Connection)
+    build_driver(uri).connection_builder(uri).call
   end
 
   private def self.build_driver(uri : URI)

src/db/database.cr

@@ -52,9 +52,10 @@ module DB
       pool_options = @driver.connection_pool_options(params)
 
       @setup_connection = ->(conn : Connection) {}
+      factory = @driver.connection_builder(@uri)
       @pool = uninitialized Pool(Connection) # in order to use self in the factory proc
       @pool = Pool.new(**pool_options) {
-        conn = @driver.build_connection(self).as(Connection)
+        conn = factory.call
         conn.auto_release = false
         conn.context = self
         @setup_connection.call conn

src/db/driver.cr

@@ -26,7 +26,11 @@ module DB
   # Refer to `Connection`, `Statement` and `ResultSet` for further
   # driver implementation instructions.
   abstract class Driver
-    abstract def build_connection(context : ConnectionContext) : Connection
+    # Returns a new connection factory.
+    #
+    # NOTE: For implementors *uri* should be parsed once. If all the options
+    # are sound a factory Proc is returned.
+    abstract def connection_builder(uri : URI) : Proc(Connection)
 
     def connection_pool_options(params : HTTP::Params)
       {