This removes the need to include reactor.hh, a source of compile
time bloat.
In some places, the call is qualified with seastar:: in order
to resolve ambiguities with a local name.
Includes are adjusted to make everything compile. We end up
having 14 translation units including reactor.hh, primarily for
deprecated things like reactor::at_exit().
Ref #1
1. Move tests to test (using singular seems to be a convention
in the rest of the code base)
2. Move boost tests to test/boost, other
(non-boost) unit tests to test/unit, tests which are
expected to be run manually to test/manual.
Update configure.py and test.py with new paths to tests.
