Extract a clean set of durability host functions

[vigoo]

After #1222, by a further refactoring step we separate the Durability implementation to a set of host functions (with access to DurableWorkerCtx) and a set of abstractions built on top of them. At the end of this step we have three layers of functionalities (still everything in golem-worker-executor-base:

• the core set of durability-related functions using DurableWorkerCtx
• the higher level abstractions built on them, used in the host functions
• the host function implementations using these higher level abstractions and calling underlying wasmtime host functions