Callosum

Features
* lower.zeromq: Add a transport option to attach monitors for logging underlying socket events ([17](https://github.com/lablup/callosum/issues/17))

Fixes
* Improve `zsock_opts` handling when explicitly specified by the library users, as previously it caused argument errors in binders and connectors ([18](https://github.com/lablup/callosum/issues/18))

Features
* Provide `repr()` of exceptions in RPC user/internal errors for better application-level error logging ([15](https://github.com/lablup/callosum/issues/15))

Features
* `upper.rpc`: Support server-side cancellation and propagation to clients by adding the `CANCELLED` RPC message type ([14](https://github.com/lablup/callosum/issues/14))

Deprecations
* Drop use of aiojobs in favor of native semaphores for limiting the task concurrency and less clutters on job scheduling semantics ([14](https://github.com/lablup/callosum/issues/14))

Fixes
* Stability updates for the RPC layer: ([14](https://github.com/lablup/callosum/issues/14))
- Fix wrong message sequence calcuation with `SEQ_BITS` and clarify the roles of `cleanup()` and `cancel()` methods in the schedulers.
- Now we use the exit-ordered scheduler by default.

Fixes
* lower.zeromq: Use destroy() for zmq context termination to improve stability and shutdown open socekts cleanly ([13](https://github.com/lablup/callosum/issues/13))

Fixes
* Fix a race condition due to overlapping RPC message sequence IDs by separating server/client message sequence IDs [(12)](https://github.com/lablup/callosum/issues/12)

Miscellaneous
* Adopt towncrier for changelog management [(11)](https://github.com/lablup/callosum/issues/11)

* FIX: Mitigate bogus KeyError when cleaning up task futures in RPC servers that has caused event
loop spinning.