Make it possible to specify interval as float which allows to use also fractions of a second.

It is usable for activity nodes where we need activity processing (i.e.
tic) to happen much faster that normally (default is 5 seconds).

It has been successfully used in a production system for months (and big dev instance)  despite not done in generic line now proposed.

@nexedi , any objections for not having this in a generic ERP5's timerservice?

/reviewed-on !562

product/TimerService/timerserver/timerserver/component.xml

@@ -4,9 +4,9 @@
   <sectiontype name="timer-server"
                datatype="timerserver.TimerServerFactory"
                implements="ZServer.server">
-    <key name="interval" datatype="integer" default="600">
+    <key name="interval" datatype="float" default="600">
       <description>
-	Interval in seconds.
+	Interval in seconds. Supports fractions of a second.
       </description>
     </key>
   </sectiontype>