simln-lib: add htlc interceptor for simulated nodes

[elnosh]

for #255

Mostly took changes from these 2 commits to add an Interceptor trait:

• carlaKC@c5d7649
• carlaKC@228706c

Some of the things I changed from those commits:

• intercept_htlc now returns a Result to communicate the result of the intercepted htlc instead of sending it through a channel.
• spawns a task in a JoinSet for each interceptor's intercept_htlc so that if any of those holds the htlc for a long time it does not block the other interceptors. It then waits for the completion of the tasks in the joinset. If any of them returns a result to fail the htlc, it drops the JoinSet since there is no need to wait for completion of other tasks because the htlc will fail anyways.

[carlaKC]

Driveby comment: could we split up the logic adding a unique index and the interceptor into separate commits?
For the sake of breaking up into smaller logical chunks / reviewability.