Wire up the selection and result-set infrastructure to consume the
extended collection wire format introduced in the previous patch and
expose per-element timestamps and TTLs to the expression evaluator.
* Add collection_cell_metadata: maps from raw element-key bytes to
timestamp and remaining TTL, one entry per collection or UDT cell.
Add a corresponding collection_element_metadata span to
evaluation_inputs so that evaluators can access it.
* Add a flag _collect_collection_timestamps to selection (selection.hh/cc).
When any selected expression contains a WRITETIME(col[key])/TTL(col[key])
or WRITETIME(col.field)/TTL(col.field) attribute, the flag is set and
the send_collection_timestamps partition-slice option is enabled,
causing storage nodes to use the extended wire format from the
previous patch.
* Implement result_set_builder::add_collection() (selection.cc): when
_collect_collection_timestamps is set, parse the extended format,
decode per-element timestamps and remaining TTLs (computed from the
stored expiry time and the query time), and store them in
_collection_element_metadata indexed by column position. When the
flag is not set, the existing plain-bytes path is unchanged.
After this patch, the new selection feature is still not available to
the end-user because the prepare step still forbids it. The next patch
will finally complete the expression preparation and evaluation.
It will read the new collection_element_metadata and return the correct
timestamp or TTL value.
