Add integrity test for huge writes. Fix speed calc problem.

Add integrity test for huge writes. Fix speed calc problem.

This commit

* Fixes a speed calculation on the integrity tests (we were
  assuming total transferred bytes was the size of the disk,
  which it is for throughputtest but is not for the integrity
  test)

* Adds a binary blob integrityhuge-test.tr, which contains
  instructions to do huge oversize reads and writes with
  flushes over a 50MB disk. This is added as a binary blob
  so everyone has the same data.

* Adds make-integrityhuge.c - a simple program which will make
  the above binary blob (though as it uses random data it will
  not be the same each time)

* Adds an /integrity_huge test to "make check"

* Modifies integritytest() to use properly buffered I/O so it
  does not deadlock.

* Modifies integritytest() to keep handles in a hash table
  to avoid spurious memory accesses if the server returns
  a duff handle.