Make map and map_with_path_builder of sync version WALs returns a reader backed by unsync::Arena

[al8n]

Suppose a sync version of sync::OrderWal is constructed by map or map_with_path_builder. In that case, there is no reason to use rarena_allocator::sync::Arena as the backend, because map or map_with_path_builder means that the OrderWal cannot be modified anymore so that no data race will happen anymore.

Instance constructed by map or map_with_path_builder is not the same as reader methods of sync::OrderWal. reader let users create a read-only view on an active log, which means the writer can still modify the active log, so a rarena_allocator::sync::Arena is necessary for thread-safe.

[al8n]

Blocked by al8n/rarena#14