In preparation for caching index objects, manage them under LSA.
Implementation notes:
key_view was changed to be a view on managed_bytes_view instead of
bytes, so it now can be fragmented. Old users of key_view now have to
linearize it. Actual linearization should be rare since partition
keys are typically small.
Index parser is now not constructing the index_entry directly, but
produces value objects which live in the standard allocator space:
class parsed_promoted_index_entry;
calss parsed_partition_index_entry;
This change was needed to support consumers which don't populate the
partition index cache and don't use LSA,
e.g. sstable::generate_summary(). It's now consumer's responsibility
to allocate index_entry out of parsed_partition_index_entry.
06e373e272