Tree nodes now hold entries at the form
{Key, ?TOMBSTONE
| BinValue
| {?TOMBSTONE, TStamp}
| {BinValue, TStamp}}
We use the form without TStamp when expiry_secs
is unset or set to 0 (i.e., values don't expire).
merger/writer: Move KV count into writer, because
now the writer:add determines if a value is expired
and thus wither a value is actually written. Thus,
writer now has a new API function which returns the
KV count written so far.
reader: lookup/fold API hides the TStamp tuples,
so only the next_node API used by the merger
is exposed to these {Key, {_, TStamp}} entries.
nursery: like reader, the TStamp'ed tuples are
not exposed in the client API; expired values
are simply not returned from fold/lookup.
hanoidb: add config option {expiry_secs, N}.
other modules: Make sure that config is passed
all the way down through (sub) processes to be
able to utilize the config option everywhere.
test: update to work with new option.
