RocksDB multiple connects are failing with error

[graninas]

See the labyrinth game (commit 7d47e83).

Scenario:

1. Type save 0. A new connection is opened, and the game will be successfully saved.
2. Type load 0. A new connection will fail.

> save 0
Game successully saved to KV DB.
> load 0
labyrinth: InvalidOperation "DBError SystemError \"user error (open: IO error: lock ./app/labyrinth/labyrinths.rdb/LOCK: No locks available)\""
labyrinth: thread blocked indefinitely in an STM transaction