cli: add COCKROACH_MEMPROF_INTERVAL
cli: add COCKROACH_MEMPROF_INTERVAL

The COCKROACH_MEMPROF_INTERVAL env var contains a duration which
indicates how often memory profiles will be generated. Generated
profiles are written to the logs directory with the prefix "memprof."
and a date/time suffix. Periodic memory profiling is only initiated for
the `start` command.

See #7018.

petermattis/cockroach,cockroachdb/cockroach,glycerine/cockroach,sunya123/cockroach,sunya123/cockroach,paperstreet/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,glycerine/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,sunya123/cockroach,vaibhavarora/cockroach,maxlang/cockroach,sunya123/cockroach,maxlang/cockroach,veteranlu/cockroach,petermattis/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,arjun4084346/cockroach,paperstreet/cockroach,petermattis/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,petermattis/cockroach,veteranlu/cockroach,arjun4084346/cockroach,maxlang/cockroach,vaibhavarora/cockroach,petermattis/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,paperstreet/cockroach,sunya123/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,maxlang/cockroach,arjun4084346/cockroach,glycerine/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,sunya123/cockroach,glycerine/cockroach,maxlang/cockroach,cockroachdb/cockroach,paperstreet/cockroach,glycerine/cockroach,veteranlu/cockroach,maxlang/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,veteranlu/cockroach,glycerine/cockroach,veteranlu/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,sunya123/cockroach,glycerine/cockroach,paperstreet/cockroach,maxlang/cockroach,petermattis/cockroach,paperstreet/cockroach,maxlang/cockroach,glycerine/cockroach,petermattis/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,sunya123/cockroach,veteranlu/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,petermattis/cockroach,cockroachdb/cockroach