design doc for Transaction

[flaneur2020]

for a KV engine, implementing ACID transaction helps provides a solid foundation for building more complex systems on top of the it.

this is a complex work, we can have a doc about the design of transaction, which allows some in-depth discussion under it.

this design doc is expected to have:

• goals & possible usages
• investigation about the design of the existed kv systems like rocksdb / badger kv
• possible solutions and trade offs

[flaneur2020]

@criccomini can you help assign this issue to me? i'd happy to make a draft in the vacation days :)