[Ready for Review] Support more convenient foreach variable/value search-ability.

[darinyu]

In this change, we

1. add values of foreach items to flowspec for convenient access to values in foreach steps.
2. Update foreach stack to include value.
3. Add formatted foreach stack (path from root to current level) to metadata, guarded by a flag.

Currently, we try to get "value" of the for each in the following order:

1. If the item is list of primitive types like ["a", "b", "c"], we will use the literal value directly
2. Otherwise, we use replib to get a serialized value of the iterable, limited to 30 charactors.

The end result will be of the form [value]. We will then

1. Use the list of value (generated in a split/parent node) in the task_finish function inside a custom decorator.
2. Use the list of value when constructing foreach stack, which will eventually be used to construct the for-loop path in metadata.

An example for the updated search UI is like the following.

[romain-intel]

why is this needed? Isn't the presence of the value backward compatible with the old code?

[darinyu]

for a node, we may want to quickly get the finish_id of the sibling node and check if they all finished (can check usage of finished_id within _queue_task_join). It is non-trivial to get value of the sibling, so we may as well not compare value at all (afterall, the index of the foreach already identifies the node).

[romain-intel]

LGTM with a small typo. Let's wait for someone from OB but we can merge afterwards.