fiber: initialize thread-local cord on demand

We're planning to introduce a basic C API for user read views (EE-only).
Like all other box C API functions, the new API functions will use the
existing box error C API for reporting errors. The problem is that
a read view created using C API should be usable from user threads
(started with the pthread lib) while the box error C API doesn't work
in user threads, because those threads don't have the cord pointer
initialized (a diagnostic area is stored in a cord object).

To address this issue, let's create a new cord object automatically on
first use of cord() if it wasn't created explicitly. Automatically
created object is destroyed at thread exit (to achieve that, we use
the C++ RAII concept).

Closes #7814

NO_DOC=The C API documentation doesn't say anything about threads.
       Let's keep it this way for now. We're planning to introduce
       a new C API to work with threads in C modules. We'll update
       the doc when it's ready.