Performance: Use const references for string statistics min and max to avoid copy construction

Via Callgrind Performance Profiling of a scenario of a Copy (Full Read and then Full Write) of a 1.9 million row ZLib Compressed ORC Table.  The #4 Usage of CPU is the std::string alloc from being called by: orc::StringColumnStatisticsImpl::update method due to the getMax/getMin calls causing std:string alloc/copy/delete.

Changing the getMaximum/getMinimum methods to return const vals will prevent these alloc/copy/deletes from occurring.

Currently with 1.6.X master the performance profile of this scenario is:

Instructions Executed: 16.6 Billion Instructions

real clock time 3.91 seconds

With the fix to use consts, this improves the CPU usage by about 38% and the Clock Time about 10% to:

Instructions Executed: 12.0 Billion Instructions

real clock time 3.53 seconds

Attached JPG showing before (left) and after (right) screenshot of callgrind.