Performance Evaluation of UDFs

This commit introduces the possibility to evaluate the performance of UDFs using a simple tracing of instructions and cycles. This adds an optional dependency to PAPI. If PAPI is available the make target is build.

Below the text from the README as an entry point for discussions if this makes sense to integrate or not: 

**Evaluating UDFs**

Using custom UDFs can incur additional overhead to the query execution as they are likely to be executed millions of times. Thus, careful optimizations of such functions is required. This sample package  allows you to quickly evaluate the performance of you UDFs by counting instructions and cycles. 

To use performance evaluation, install PAPI for your local platform and adapt the following bit of code according to your own UDFs.

    #include "helper/papi-tracer.h"
    #include "helper/udf-execute.h"
    
    // ...
    impala_udf::UdfExecuteHelper executor;
    { 
      ScopedTracer sct("AddUdf", 1000u);
      for(int i=0; i < sct.numCalls(); ++i) {
        counter += executor.ExecuteUdf<IntVal, IntVal, IntVal>(
          AddUdf, IntVal(i), IntVal(i+1)).val;
      }
    }


The above code will call the UDF `AddUdf` from the sample code 1000 times and record the cycles and instructions spent. To compile check the `CMakeList.txt` for the appropriate target to get all the dependencies. Once the binary is built, you can simply execute it. The output of the program should now contain something like this:

    --------------------------------------
    Performance Analysis for AddUdf
    Real Time:                 0
    Total Instructions:        64
    Instructions / Cycle:        2.53008
    Cycles / Call:             25
    Time in s per 10^9 calls @3GhZ: 8
    Time in s per 10^9 calls @2GhZ: 12

This overview tells you how many instructions were executed per call, what the ratio between instructions and cycles is, and most importantly what the runtime probably will be when the UDF is execute 1B times.

Two things are very important for optimizing UDF code: the ratio of instructions per cycle should be as high as possible (on modern Intel platforms the theoretical limit is 4). In addition the number of instructions should be as low as possible. Both measures determine together the overall execution time.