Compaction: Allow snooze_period to be a float

We are experiencing an issue similar to #1579, partly because we have
dozens of thousands of databases.

On our servers:
- `snooze_period = 0` leads to a significant CPU load,
- `snooze_period = 1` is too long for compaction to finish within 20
  hours (after 20 hours, compaction is stopped to allow other
  CPU-intensive processes to run, and when compaction restarts, it does
  not pick up where it left 4 hours earlier -- by the way proposal at
  #1775 would be really great to fix that!)

This commits allows to write `snooze_period = 0.3` in configuration.

rel/overlay/etc/default.ini

@@ -435,7 +435,7 @@ min_file_size = 131072
 ; databases, the compaction_daemon can create significant CPU load when
 ; checking whether databases and view indexes need compacting. The
 ; snooze_period setting ensures a smoother CPU load. Defaults to
-; 3 seconds wait.
+; 3 seconds wait. Can be a float (e.g. write 0.3 for 300 ms).
 ; snooze_period = 3
 
 [compactions]

src/couch/src/couch_compaction_daemon.erl

@@ -126,7 +126,7 @@ handle_config_terminate(_Server, _Reason, _State) ->
     erlang:send_after(?RELISTEN_DELAY, whereis(?MODULE), restart_config_listener).
 
 compact_loop(Parent) ->
-    SnoozePeriod = config:get_integer("compaction_daemon", "snooze_period", 3),
+    SnoozePeriod = config:get_float("compaction_daemon", "snooze_period", 3.0),
     {ok, _} = couch_server:all_databases(
         fun(DbName, Acc) ->
             case ets:info(?CONFIG_ETS, size) =:= 0 of
@@ -140,7 +140,7 @@ compact_loop(Parent) ->
                     case check_period(Config) of
                     true ->
                         maybe_compact_db(Parent, DbName, Config),
-                        ok = timer:sleep(SnoozePeriod * 1000);
+                        ok = timer:sleep(trunc(SnoozePeriod * 1000));
                     false ->
                         ok
                     end
@@ -231,8 +231,8 @@ maybe_compact_views(DbName, [DDocName | Rest], Config) ->
         timeout ->
             ok
         end,
-        SnoozePeriod = config:get_integer("compaction_daemon", "snooze_period", 3),
-        ok = timer:sleep(SnoozePeriod * 1000);
+        SnoozePeriod = config:get_float("compaction_daemon", "snooze_period", 3.0),
+        ok = timer:sleep(trunc(SnoozePeriod * 1000));
     false ->
         ok
     end.