compact_query: Rename partition_limit
compact_query: Rename partition_limit

This patch renames compact_query::_partition_limit to
_current_partition_limit for clarity, as the next patch adds
a partition limit that limits the number of partitions.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

duarten/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,kjniemi/scylla