Fix fsearch by using a sharded file buffer

Each worker thread has it own stringstream per ShardedFileBuffer. All streams are joined and flushed to the file when a ShardedFileBuffer is destructed.

Flushing a ShardedFileBuffer is not thread safe and must not be called concurrently to any modifications to the ShardedFileBuffer.

Use a ShardedFileBuffer to acumulate and print all paths during fsearch.

Closes !351 (closed).