RFR: 8240778: JFR: Create timer task lazily

[Erik Gahlin]

Hi,

Could I have a review of a fix that creates the JFR timer task lazily. 
It also takes care of an issue where an ISE could be thrown if a call to 
Recording::scheduleStart would happen after JFR has shutdown (not very 
likely).

Bug:
https://bugs.openjdk.java.net/browse/JDK-8240778

Webrev:
http://cr.openjdk.java.net/~egahlin/8240778/

Testing:
jdk/jdk/jfr

Thanks
Erik