Plasma DirectIO is broken

If directIO flag is set, log cleaner is throwing errors for a bit then crashing

Can be replicated by setting cfg.IOMode = DirectIO in the plasma tests

— FAIL: TestGMRecovery (4.89s)

panic: read teststore.data/log.00000000000000.data: invalid argument [recovered]

panic: read teststore.data/log.00000000000000.data: invalid argument

goroutine 12 [running]:

testing.tRunner.func1(0xc420112000)

/usr/local/go/src/testing/testing.go:742 +0x29d

panic(0x6f8940, 0xc420711500)

/usr/local/go/src/runtime/panic.go:502 +0x229

github.com/couchbase/plasma.newTestIntPlasmaStore(0xc8, 0x190, 0x19, 0x0, 0x777318, 0x777350, 0x0, 0x0, 0x0, 0x0, ...)

/lvmdrive/couchbase/goproj/src/github.com/couchbase/plasma/plasma_test.go:49 +0xbb

github.com/couchbase/plasma.TestGMRecovery(0xc420112000)

/lvmdrive/couchbase/goproj/src/github.com/couchbase/plasma/gm_test.go:61 +0x631

testing.tRunner(0xc420112000, 0x777130)

/usr/local/go/src/testing/testing.go:777 +0xd0

created by testing.(*T).Run

/usr/local/go/src/testing/testing.go:824 +0x2e0

exit status 2

Seems to be caused by this -commit https://github.com/couchbase/plasma/commit/d54566388b23a3fe15b3f0382070fc8ce1338aec