Prototype for context shared between timers

[AssemblyJohn]

In this PR a small modification has been made, to allow multiple timers to run from the same asio::io_context, from a unique thread instead of spawning a different thread for each timer.

1. The code will keep backwards compatibility, no application re-design would be required on basic usage.
2. On a custom timer that uses a boost::asio::io_context* io_context constructor an extra work guard should be placed around the provided io_context from exterior code, the custom ctor will not take care of that

[AssemblyJohn]

This is a draft PR that does not solve some issues that are open to discussion:

1. Using a static singleton shared between all timer instances, in case the 'SharedContext' template param is set to true. Alternatives to the singleton pattern to be considered
2. Code wait issues. If user code uses thread::sleep or conditional variables or other blocking functions from a timer callback major disruptions in callback invocation can occur. It presumes that any work done in timer callbacks is quickly done and the function returns