All reader are soon going to require a valid permit, so make sure we
have a valid permit which we can pass to the delegate reader when
creating it. This means `memtable::make_flat_reader()` now also requires
a permit to be passed to it.
Internally the permit is stored in `scanning_reader`, which is used both
for flushes and normal reads. In the former case a permit is not
required.
The plan is to move the unstructured content of tests/ directory
into the following directories of test/:
test/lib - shared header and source files for unit tests
test/boost - boost unit tests
test/unit - non-boost unit tests
test/manual - tests intended to be run manually
test/resource - binary test resources and configuration files
In order to not break git bisect and preserve the file history,
first move most of the header files and resources.
Update paths to these files in .cc files, which are not moved.
