Don't warn on 44 and 45

Ocaml 4.1 generates warnings 44 and 45 when symbols are shadowed.
This can potentially be useful, but Batteries works by shadowing a
large number of symbols from the prelude, so this only generates a lot
of noise.
Implement SQL pool

The SQL pool is an alternate storage format for the backup pools.
Rather than having to maintain our own index and file formats, store
the bulk of the information in an sqlite3 database.  Blobs that take
more than 100k are written to individual files, as per advice from the
sqlite3 docs.

The pool type is auto-detected upon open, and there are two create
commands, the original for the older file-based pool, and create-sql
which will create a new SQL pool.

The seendb is still kept in separate files, now under poolname.seen
along side the database.
Use Pool_util in manager

Instead of trying to guess whether a pool can be opened, just try.
Move sql pool into a dir

Rather than a file and some dirs at the top level, move the database
into a directory itself.  The resulting format is

   poolname/                       - The directory with the pool
   poolname/data.db                - The SQL database
   poolname/blobs/xx/xxx           - Expanded blobs
   poolname/seen/cache-xxx.sqlite  - The "seen" databases.