Add thread id to the Connection class

[pnbabu]

Currently, the send() function in the Connection class is the only place where the thread_id of the connection is available. This issue is to propose adding a new variable, thread_id to the Connection class as a private member and a corresponding private function get_thread() that returns the thread id. This makes it easier to query the thread_id from all the functions in the synapse models, along with the models written in NESTML.

The idea is to introduce a new variable unsigned short int thread_id in the Connection class. The connection object has a target identifier target_ is 16 bits + 16 bits of padding + 32 bits of SynIdDelay data structure. The 16 bits of padding could be used for the connection's thread ID.

[github-actions]

Issue automatically marked stale!

[pnbabu]

As mentioned in #2921, the thread_id can be accessed via kernel().vp_manager.get_thread_id() from the synapse. A new issue is opened for this in NESTML: nest/nestml#1158. Closing this issue for now.