Improved and cleaned up the bucket library. The original "single operation

Improved and cleaned up the bucket library. The original "single operation
pending per process" invariant was no longer feasible (and it caused several
problems in Shepherd).

Reading and writing of buckets now uses dynamically allocated fastbufs and
there can be any number of readers at any time, but only a single writer
(otherwise a deadlock would occur). Read streams are seekable, write streams
at least btell()-able.

Also removed the omnipresent global variables for start of current bucket
etc., each part (Find, Slurp, Create, Shakedown, ...) has its own state
variables.

Added some more sanity checks.