Improve handling of directories with many files

bbackupd : When a directory has lots of files, there are rather too many compares going on when searching for it's entry in the directory listing retrieved from the server. This scales logarithmically.

bbstored : When housekeeping, bbstored reads every directory. Within each directory, the contents are only scanned linearly a couple of times, so overall it should scale linearly with the number of directories (where number of files in them are far less important).

We do need to move to a ref counted store, to avoid all this scanning. But on review of the code, I don't think the excessive CPU usage on the server is due to inefficient handling of large directories.

bbackupd gets slower and slower when backing up a directory with many files. The problem appears to be that the directory is rewritten after each file is added, which is O(n^{2) in number of files in the directory.}