Allow DB::Pool to be used a generic connection pool (#131)

* Allow DB::Pool to be a generic connection pool

* Use fully qualified class name for consistency

Co-authored-by: Brian J. Cardiff <bcardiff@gmail.com>

* Wrap only the necessary code in an `ensure`

* Add spec for http client pool

* Fix ICE in crystal-sqlite3

Co-authored-by: Brian J. Cardiff <bcardiff@gmail.com>

spec/support/http.cr

@@ -0,0 +1,48 @@
+require "http"
+require "./fibers"
+
+def wait_for(timeout = 5.seconds)
+  now = Time.monotonic
+
+  until yield
+    Fiber.yield
+
+    if (Time.monotonic - now) > timeout
+      raise "block failed to evaluate to true within #{timeout}"
+    end
+  end
+end
+
+# Helper method which runs *server*
+# 1. Spawns `server.listen` in a new fiber.
+# 2. Waits until `server.listening?`.
+# 3. Yields to the given block.
+# 4. Ensures the server is closed.
+# 5. After returning from the block, it waits for the server to gracefully
+#    shut down before continuing execution in the current fiber.
+# 6. If the listening fiber raises an exception, it is rescued and re-raised
+#    in the current fiber.
+def run_server(server)
+  server_done = Channel(Exception?).new
+
+  f = spawn do
+    server.listen
+  rescue exc
+    server_done.send exc
+  else
+    server_done.send nil
+  end
+
+  begin
+    wait_for { server.listening? }
+    wait_until_blocked f
+
+    yield server_done
+  ensure
+    server.close unless server.closed?
+
+    if exc = server_done.receive
+      raise exc
+    end
+  end
+end