RFR: 8185525: [Event Request] Add Tracing event for DictionarySizes

[gerard ziemski]

hi all,

Please review this, hopefully final, fix. After some offline discussion, 
we agreed to:

#1 Keep the rate attributes (can't synthesize otherwise because we don't 
track additions/deletions on their own)

#2 Use data from last event if we can't lock a table (fulfills JFR 
contract in which we need to deliver event at specified period)

Bug:     https://bugs.openjdk.java.net/browse/JDK-8185525
Webrev:  http://cr.openjdk.java.net/~gziemski/8185525_rev9
Testing: Mach5 tier1,2,3 (another Mach5 tier1,2,3,4,5,6,7 in progress…)

cheers