[EchoSever] implement random computations variants

Now three variants of computation are available:

* fixed (echoserver <port> <computation>:
   This will always consume computation us before sending the echo
   back to the client.
* random range (echoserver <port> <computation> <computation-max>:
   This will consume a random computation uniformly selected
   from the interval [computation, computation-max] us.
* random min-max (echoserver <port> <computation> <computation-max> <max-probability>
   This will either consume computation or computation-max us.
   The max computation is randomly chosen with the specified probability.

All random values are generated with a thread_local mt19937 generator
and uniformly distributed with uniform_{int,real}_distribution.
27 jobs for random-computation-echoserver in 35 minutes and 5 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Smoke Test
passed clang-tidy #459351

00:21:52

passed fast-static-analysis #459349

00:02:16

passed iwyu #459350

00:08:24

passed smoke-test-suite #459348

00:02:48

 
  Test
passed test-clang #459355

00:02:22

passed test-clang-debug #459357

00:04:26

passed test-clang-release #459356

00:13:08

passed test-clang-sanitizer-undefined #459361

00:06:05

passed test-do-not-log-timestamp #459366

00:05:47

passed test-futex-wakeup-semaphore #459368

00:02:32

passed test-gcc #459352

00:04:18

passed test-gcc-debug #459354

00:13:08

passed test-gcc-release #459353

00:02:30

passed test-laws #459362

00:02:50

passed test-laws-release #459363

00:02:30

passed test-locked-wakeup-semaphore #459369

00:02:55

passed test-locked-ws-queues #459367

00:02:35

passed test-mmapped-log #459371

00:03:05

passed test-pipe-sleep-strategy #459373

00:02:18

passed test-pipe-sleep-strategy-no-completer #459374

00:03:02

passed test-set-affinity-on-block #459370

00:03:17

passed test-single-uring #459372

00:01:57

passed test-with-stats #459359

00:03:16

passed test-with-userspace-rcu #459360

00:03:28

passed test-worker-no-sleep #459358

00:02:51

passed test-worker-wakeup-strategy-all #459364

00:01:51

passed test-worker-wakeup-strategy-throttle #459365

00:03:33