Basement Tombstones

Further progress towards the tidying up of basement tombstones in the
Ledger, with support added for key-listing to help with testing (and as
a potentially required feature).

The test is incomplete, but committing at this stage as the last commit
broke some tests (within the test code).

There are some outstanding questions about the handling of tombstones in
the Journal during compaction.  There exists a condition whereby values
could return if a recent journal is compacted and tombstones are removed
(as they are no longer present), but older journals have not been
compacted.  Now on stop/start - if the Ledger is wiped the removal of
the keys will be forgotten but the original PUTs would still remain.

The safest thing maybe to have rule that tombstones are never deleted
from the Inker's Journal - and accept the build-up of garbage.  Or there
could be an addition to the compaction process that checks back through
all the inker files to check that the Key of a tombstone is not present
in the past, before it is removed in the compaction.

test/end_to_end/testutil.erl

@@ -28,7 +28,7 @@
 
 reset_filestructure() ->
     % io:format("Waiting ~w ms to give a chance for all file closes " ++
-                "to complete~n", [Wait]),
+    %             "to complete~n", [Wait]),
     % timer:sleep(Wait),
     RootPath  = "test",
     filelib:ensure_dir(RootPath ++ "/journal/"),