Redirect rebalance observer logs to a different file.

Most often the ns_server.debug.log wraps around if there is a stuck rebalance or a rebalance that hasn't made progress in a really long time and it ends up being completely filled with the following logs (which often doesn't add much value expect showing there is no document movement).

[ns_server:debug,2023-11-03T10:50:35.479+07:00,ns_1@bdgpt2chd201.tc.xl.co.id:<0.8188.2071>:ns_rebalance_observer:docs_left_updater_loop:507]Starting docs_left_updater_loop:"com.amdocs.digital.ms.productorder.futurelauncher

"

[{0,

  {vbucket_info,

....

We should consider moving these logs to a separate file (or the new ns_server.trace.log file) to preserve other relevant logs that might be useful for debugging an issue. For example in CBSE-15670 it would have been nice to see the vbmap logs to see why a unbalanced map was generated (albeit in that particular case we had hints from the config as to why the map was unbalance, but we might not be as lucky in other cases).