Support of sequential execution

[roggervalf]

There are cases where a job should be processed after another one. That requires to keep kind of sort of ordering but considering all our current states (waiting, prioritized, delayed, waiting-children, completed, failed and paused).
This new concept named as "Chains" implies to execute a particular job after another one only when previous job is already finished. This is similar to our waiting-children state, but used as creating a child and then a parent.
Something to consider is that we need to track which is the last possible child in the chain so a new parent job can be added and a relationship can be made.
If a child is already finished, remove that reference from the chain key.

taskforcesh/bullmq#2774
taskforcesh/bullmq#2647
taskforcesh/bullmq#2630
taskforcesh/bullmq#1858
taskforcesh/bullmq#1309
#68

previous approaches:
taskforcesh/bullmq#2465
taskforcesh/bullmq#2687