feat: Add a function to check if the instance is the only instance

[ouuan]

The function: bool SingleApplication::isSingleInstance()
Returns if the instance is the only running instance.

It is hard to check the number of running instances after a crash,
so this is an alternative solution for #89.

It uses a trick: the shared memory will be released after the last
instance detaches from it.

This closes #89.

[itay-grudev]

While this is a very creative solution, it creates a problem. For a small duration while the check is being performed, an instance can be started as a primary. So basically this function would be considered extremely dangerous and breaking the library.

I am much more inclined to accept a solution that pings existing instances to count how many are alive. I imagine it working like this:

0. All instance sockets when using secondary instances are kept alive.
1. You call countActiveInstances()
2. It pings all sockets with a few bytes of random data
3. Cleans up sockets with errors
4. Counts sockets with a correct response
5. Returns that number