[8u] RFR 8229022: BufferedReader performance can be improved by using StringBuilder

[Aleksey Shipilev]

Original fix:
  https://bugs.openjdk.java.net/browse/JDK-8229022
  https://hg.openjdk.java.net/jdk/jdk/rev/ed0058d06107

Patch does not apply cleanly to 8u, because context is different. Reapplying hunks by hand yield
this 8u webrev:
  https://cr.openjdk.java.net/~shade/8229022/webrev.8u.01/

Testing: tier1

-- 
Thanks,
-Aleksey