Add shared mutex

[Clcanny]

This pull request introduces a minimal yet functional implementation of async_shared_mutex.

[MaxMotovilov]

Sorry for a long delay in review - I have just assumed the co-maintainer's duties and will try to look through PR backlog, among other things.

Questions regarding this PR:

• IIUC, the goal is to introduce a new functional component similar to a reader/writer lock (can either take out multiple shared locks or one non-shared lock)? It would be nice to include a concise description of the new API somewhere in the PR - ideally in the docs (doc/api_reference.md) :-)
• I am presently working on merging a PR that reimplements async_mutex in order to make it cancellable. At first glance, your implementation relies on async_mutex directly and should not prevent this desirable trait from propagating to your senders. However
  • You'll need to set the sends_done flag to true (or to the value from async_mutex sender, to be pedantic), and
  • It would be advisable to include unit tests verifying the cancellability of the resulting senders
• Finally and most importantly - your implementation of async_shared_mutex::try_enqueue() relies on sync_wait() on the embedded async_mutex instance. This looks like a showstopper as it would potentially block the scheduler's thread for an indefinitely long time. Is there an alternative strategy?