* filer: bound TraverseBfsMetadata memory by queuing directory paths
The BFS enqueued every entry, so it held the whole subtree in memory
including each file's chunk list. A filer serving a peer's first-time
bootstrap traversal of a large tree could exhaust memory and get killed.
Stream each entry as it is visited and queue only directory paths to
descend into. Memory is now bounded by the number of directories rather
than the entire tree, and the streamed output order is unchanged.
* filer: match excluded prefixes on path-component boundaries
Only treat an excluded prefix as a match when it ends at a path
boundary, so excluding /a/b does not also drop a sibling like /a/bc.
Short-circuit the trie walk on the first real match.
Chris Lu