hammer2 - Implement variable-sized indirect blocks, clustered reads

hammer2 - Implement variable-sized indirect blocks, clustered reads

* Implement 4K indirect blocks as well as 64K indirect blocks

* Do not flush the chains for VOP_FSYNC() calls issued from 'sync' or
  from the syncer, only flush the chains when called from fsync().

  This solves a fairly nasty repeated copy-on-write ops on the same block
  in the topology.

* Issue appropriate cluster_read() calls instead of bread().

* Implement forward-looking bmap across multiple blocks instead of just one.