Why not reuse seastar framework?
Can you describe your distributed log thing? Is it like facebook-logdevice or apache-bookeeper?
With multi-threading you need to think about all things holistically. How you handle backpressure, how you do snapshotting. How you implement multi-key operations or blocking transactions. So you need special algorithms to provide atomicity, you need fibers/coroutines to be able to block your calling context yet unblock the cpu for other tasks etc. All this was designed bottom up from scratch. Seastar could work theoretically but I am not a fan of coding style with futures and continuations - they are pretty confusing, especially in C++. My choice was using fibers - which provide more natural way of writing code.
I have not designed the distributed long thingy. Will do it in the next 2 months.