RFR JDK-8038654: Separate SymbolTable and StringTable code

[Gerard Ziemski]

hi all,

Please review this simple enhancement, which refactors symbolTable into 
symbolTable and stringTable

No new or removed code.

Webrev: http://cr.openjdk.java.net/~gziemski/8038654_rev0
Bug: https://bugs.openjdk.java.net/browse/JDK-8038654

Tested with hotspot jtreg, vm.quicklooks and JPRT. Locally builds on Mac 
OS X and Linux.

Thank you.