log request processing time in rasserver log

[Dimitar Misev]

Currently in rasserver log lines for each request look like this:

 [INFO] - 2019-10-10 11:29:06,786: Request: 'SELECT c FROM test_rgb AS c'... parsing... checking semantics... evaluating... result type 'set <marray <char, [0:78]>>', 1 element(s)... ok, transferred 151 bytes.

It will be good to log also here how long it took to evaluate the query, e.g.

 [INFO] - 2019-10-10 11:29:06,786: Request: 'SELECT c FROM test_rgb AS c'... parsing... checking semantics... evaluating... result type 'set <marray <char, [0:78]>>', 1 element(s)... ok, execution time X ms, result size 151 bytes.

This should be done in servercomm.cc whereever the "transferred .. bytes" is currently logged. Use common/util/timer.hh for measuring elapsed time since starting the query evaluation to when the result is returned.