Also, don't set an unsigned to -1…

* Add optional throttle Semaphore pointer to limit the number
  of spawned fn as well as directory walk fibers
* Use const references to the passed functions instead of values
* fsearch: Use max_running as fn and recursion throttle

* Pass the IO results on the stack instead of storing them in the
  client object.
* Terminate the runtime on quit to print stats.
* Free Client objects.

Each worker thread has it own stringstream per ShardedFileBuffer.
All streams are joined and flushed to the file when a ShardedFileBuffer
is destructed.

Flushing a ShardedFileBuffer is not thread-safe and must not be called
concurrently to any modifications to the ShardedFileBuffer.

Use a ShardedFileBuffer to accumulate and print all paths during fsearch.

In the emper-fs-eval I would like to validate the output of the fsearch
variants against a list of known files. Not writing any log messages to
stdout make that easier.

Use -1 as the offset of the write to stdout.
Passing offset -1 means to use the file's implicit offset just like write(2).
When using a tty as stdout offset 0 is fine but when redirecting stdout
to a file 0 will cause that previously found paths will always be
overwritten.

Thanks to Nicolas Pfeiffer for writing the initial prototypical
implementation of continuation stealing and the cactus stack
mechanism, on which this is based.

Co-authored-by: Nicolas Pfeiffer <pfeiffer@cs.fau.de>

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.

This is needed by emper-io-eval because apparently our startup/termination
times are shorter than the OS allows the rebinding of the same tcp tuple.

Also make all global variables static because they don't have to be exported.

The Coordinator is used for our echo evaluation and implements
a barrier style synchronization mechanism for processes spread across the network.

Check if std::filesystem::recursive_directory_iterator and std::filesystem::path
are available before using those in EMPER code.

We do not export the symbols using the not supported filesystem features
in our public headers using preprocessor ifdef.

But the code in the cpp files using it not removed using the preprocessor.
To allow linkage we use a constexpr which throws a logic_error on runtime
rendering the rest of the code dead und thus prevents its generation by
the compiler.
This methods allows the compiler to see the code in its analysis passes
but does not fail during linking.

Allow meson.build files in emper/ subdirectories add configuration options
by consuming the conf_data object after all subdirectories were visited.

Introduce a quasi naming standard for cpp feature flags in meson code:
	cpp_has_<namespace>_<feature>

Examples:
	cpp_has_fs_path