Implement get_provisioners optimisation of the stake contract

[miloszm]

Summary

Current stake contract's method get_provisioners is a performance bottleneck, as its processing time grows linearly with the number of provisioners. According to measurements, the following data illustrates the problem:

number of provisioners: processing time:
60......................................228.988ms
600....................................2.300383s
1000...................................3.648217s
10000.................................33.811937s

get provisioners becomes a performance bottleneck and needs to be improved

Possible solution design or implementation

Easy optimisation brings significant improvements. We should keep PublicKey in our data structure rather than convert the bytes stored into a PublicKey every time get_provisioners is called. Such change gives the following results:

number of provisioners: processing time:
1000..................................0.002s
10000................................0.012s

Additional context

Since the change is in the stake contract, we need to plan for migration of the stake contract in order to enforce the above optimisation.

[autholykos]

The consensus needs to consume events from the Stake contract in order to make efficient use of the Provisioner information

[autholykos]

Blocked by #1448

[HDauven]

Resolved by #1459