Synchronize access to @_config only during write
Synchronize access to @_config only during write

Prior this commit read and write access to `#config` is synchronized.

This commit removes the need of synchronizing when reading an already
initialized @_config.

Writing @_config (typically during boot time) via `configure { ... }`
is still synchronized which "feels safe" (TM).

dry-rb/dry-configurable,dry-rb/dry-configurable,dryrb/dry-configurable