Draft: Client performance statistics
Add statistics gathering and reporting to utilities in tests/progs.
Create new command-line option -% filename for C programs. For the scripts, the environment variable
TESTER_STATS; if set to a filename, route statistics there by invoking the C program with the value of that variable as an argument to -%.
When writing statistics output, the client detects whether the output file is a FIFO or not. If not, the output is printed as test. If it's a FIFO, it was created by
slapd-tester.c, which reads and coordinates client statistics display. The client writes a portion of a
server struct in binary form for slapd-tester to read and display to filename.
tests/STATS.md for further details.