Adding model::no_timeout to a duration overflows

[graphcareful]

There are some places in the codebase where a model::timeout_clock::duration is accepted as parameter. In these sites this value is commonly added to model::timeout_clock::now().

The issue arises when model::no_timeout is passed as duration parameter like so:

        return create_topics(
          without_custom_assignments(std::move(topics)),
          model::timeout_clock::now() + timeout);

This overflows, which is bad because depending on the nature of the result of the addition, it may be the case that the actual timeout is a very small value. Proof of the overflow is provided in this tiny unit test:

#include <seastar/core/lowres_clock.hh>
#include <seastar/testing/thread_test_case.hh>

SEASTAR_THREAD_TEST_CASE(timer_test) {
    const auto max = seastar::lowres_clock_impl::steady_duration::max();
    const auto max_tp = seastar::lowres_clock_impl::steady_time_point::max();
    const auto now = seastar::lowres_clock_impl::steady_now();
    BOOST_CHECK((now + max) == max_tp);
}

Here is the result:

Running 1 test case...
WARNING: debug mode. Not for benchmarking or production
random-seed=2453175103
==1400454==WARNING: ASan doesn't fully support makecontext/swapcontext functions and may produce false positives in some cases!
/home/robert/workspace/redpanda/vbuild/llvm/install/bin/../include/c++/v1/chrono:1249:35: runtime error: signed integer overflow: 62891062 + 9223372036854775807 cannot be represented in type 'long long'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/robert/workspace/redpanda/vbuild/llvm/install/bin/../include/c++/v1/chrono:1249:35 in 
../../../src/v/coproc/tests/tt.cc(18): error: in "timer_test": check (now + max) == max_tp has failed

Why haven't we seen an issue with create_topics? Probably because clients always pass a sane timeout when creating a topic. However this issue was just observed with #3119 during testing and it was found that it can be an issue elsewhere in the codebase too.

JIRA Link: CORE-793

[bharathv]

Assigning to myself, seems like good first issue (and still seems relevant).