Currently, we keep the entries in a circular_buffer, which uses
a contiguous storage. For large partitions with many promoted index
entries this can cause OOM and sstable compaction failure.
A similar problem exists for the offset vector built
in write_promoted_index().
This change solves the problem by serializing promoted index entries
and the offset vector on the fly directly into a bytes_ostream, which
uses fragmented storage.
The serialization of the first entry is deferred, so that
serialization is avoided if there will be less than 2
entries. Promoted index is not added for such partitions.
There still remains a problem that large-enough promoted index can cause OOM.
Refs #4217
08f4a3664e