Revert "fixes #327"

This reverts commit a80ec2e259a60ff094a62b43032efe6a5584dd9e.

This papered over the issue of NaNs being in the index's
last_rows causing search_last_row to always be incorrect.

This can be tested by a testcase that searches for x > 0 where x values
are:

1 1 1 1 ... 1 NaN 1 NaN 1
[ chunks    ] [ LRs     ]

I don't know how to write the test case.
fix NaN issues
fix index not being used when it says there are no results #351

and change seqcache logic
optimisation -- skip _write_to_seqcache sometimes
test having two iterators and _seqcache
Merge branch 'iter_fixes2' of https://github.com/r3m0t/PyTables into r3m0t-iter_fixes2
Merge branch 'r3m0t-iter_fixes2' into develop