memtables: rename `partition_snapshot_accounter` for consistency
memtables: rename `partition_snapshot_accounter` for consistency

It is actually `partition_snapshot_flush_accounter`, as opposed to
`partition_snapshot_read_accounter`.

Signed-off-by: Michael Livshin <ae60993c65de9cdf2ca07b7426c976808535d555@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla