Data racing may happen in the current redis related implmentation

[howjmay]

The current redis implementation may cause data racing. We should investigate whether will data racing occur.

[howjmay]

There are three kinds of redis list are used in current tangle-accelerator. Data racing may happen during operation among these lists
These lists are

1. buffer list: Buffer the unsent failed transactions
2. done list: Buffer the sent failed transactions
3. bundle list: Store the bundle objects. Each element in this list is a transaction object

[howjmay]

buffer list

New UUID would be pushed to buffer list when ta_send_trytes() failed.
The elements in buffer list would be popped in broadcast_buffered_txn() when a bundle is successfully broadcasted.

[howjmay]

done list

Successefully broadcasted UUID will be pushed in broadcast_buffered_txn()
UUID will be popped in ta_fetch_txn_with_uuid(), when a client successfully fetch the information.

[howjmay]

bundle list

Bundle list will be created in the following 2 situation:

1. Push failed bundle into the list before pusing the UUID to the list in push_txn_to_buffer()
2. Push broadcasted bundle to sent bundle list in broadcast_buffered_txn(). (TODO: we should delete the UUID from the unsent transction list first, then delete the outdated bundle list)

Bundle list will be deleted at two place:

1. Buffered bundle has been successfully broadcasted, so we need to fill the bundle list with new value (trunk, branch, etc)
2. A client has fetched this buffered bundle in sent buffer list.

[howjmay]

Data racing may happen in

1. A bundle is transfered from buffer list to done list
   Between cache_list_pop() and cache_list_push() (around here https://github.com/DLTcollab/tangle-accelerator/blob/develop/accelerator/core/core.c#L741), a request may not able to find the corresponding UUID.
2. A bundle is cleared from the done list, since TA reaches the maximum capacity.
   A request want to fetch bundle object, but health_track thread interupt and clear this bundle.

CC @marktwtn