Add a block profiling mode for Lightgrep searching, enabled with a #define

This patch is only enabled if LGBENCHMARK is defined.

When enabled, the amount of time spent conducting lightgrep searches for each block will be recorded and output to stdout. No search hits will be reported to the scanners, so that the timings are fair and do not include any additional validation. Therefore, do not expect any output from lightgrep-based scanners.

This code is extremely helpful in tracking the performance of lightgrep in bulk_extractor across versions (and optimizations) and in identifying input that's problematic.