main: Lower shares for main scheduling group     The main scheduling group has the shares of 1000, which is as high as   the statement group.     From time to time, we see unexpected scheduling group leaking to the main group, which causes the drop of the query performance.   This patch reduce the main scheduling shares to 200, which is the same as the maintenance scheduling group. It is a safer default in case code leaks to the main scheduling group.          Refs: #7720
main: Lower shares for main scheduling group     The main scheduling group has the shares of 1000, which is as high as   the statement group.     From time to time, we see unexpected scheduling group leaking to the main group, which causes the drop of the query performance.   This patch reduce the main scheduling shares to 200, which is the same as the maintenance scheduling group. It is a safer default in case code leaks to the main scheduling group.          Refs: #7720

Closes #8243

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