search: deletions are not modifications

1) pkg/search: documented that deletions times do not
qualify as modtimes

2) pkg/index: got rid of DeletedAt, and keyDeletes

http://camlistore.org/issue/191

Change-Id: I39578913345454d36af4599e29e7053f46577846
localdisk: change hashing structure

Before the files were stored in directories like
sha1/012/345/sha-012345xxxxx.dat, meaning there were 4096 (16^3)
top-level directories, each with up to 4096 child directories.  We
never really did the math, and the result millions (up to 16.7
million) directories with 1 file each.

Now the hashing structure is only 256 wide (two hex digits). If we
considered 4096 files in a directory acceptable before, that means the
new scheme can go up to 256*256*4096 files (268 million), which is
about 512 times bigger than my personal Camlistore instance
now. Larger users should probably be using the diskpacked storage
backend, anyway.

On start-up, the code now migrates the old format to the new format.

Change-Id: I17f7e830c50a5b770c57ee92d51f122340a0afbb
Merge branch 'master' of https://camlistore.googlesource.com/camlistore