storage: add a lease-checker-and-expirer crawler, plus web status page.

storage: add a lease-checker-and-expirer crawler, plus web status page.

This walks slowly through all shares, examining their leases, deciding which
are still valid and which have expired. Once enabled, it will then remove the
expired leases, and delete shares which no longer have any valid leases. Note
that there is not yet a tahoe.cfg option to enable lease-deletion: the
current code is read-only. A subsequent patch will add a tahoe.cfg knob to
control this, as well as docs. Some other minor items included in this patch:

 tahoe debug dump-share has a new --leases-only flag
 storage sharefile/leaseinfo code is cleaned up
 storage web status page (/storage) has more info, more tests coverage
 space-left measurement on OS-X should be more accurate (it was off by 2048x)
  (use stat .f_frsize instead of f_bsize)