RFR (M): 8210462: Fix remaining mentions of initial mark

[Thomas Schatzl]

Hi all,

   can I have reviews for this change that removes remaining use of the 
"initial mark pause" to the "newer" concurrent start in the code?

This is mostly a straightforward s/initial mark/concurrent start/ 
replacement.

CR:
https://bugs.openjdk.java.net/browse/JDK-8210462
Webrev:
http://cr.openjdk.java.net/~tschatzl/8210462/webrev/
Testing:
tier1-5

Thanks,
   Thomas