internal/pool.Pool[T] and switch internal/query.Client from subPool to pool.Pool[Session]

Author: asmyasnikov

internal/pool/errors.go

@@ -0,0 +1,10 @@
+package pool
+
+import (
+	"errors"
+)
+
+var (
+	errClosedPool   = errors.New("closed Pool")
+	errPoolOverflow = errors.New("Pool overflow")
+)