Hub blacklist is not thread safe

[MariemBaccari]

Description (Actual behavior)

The hub's blacklist is a simple array of strings which is not thread safe since we implemented the fine grain locking of the hub as opposed to locking the entire hub at every step.

Expected behavior

The blacklist should be implemented as a thread safe structure.

Front-ends:

• Fe1-Web (please include browser's names & version)
• Fe2-Android (please specify if phone or emulation, and Android version)
• Not applicable

Back-ends:

• Be1-Go
• Be2-Scala
• Not applicable

Impact

This can cause concurrency issues.

Possible root cause

In the Hub struct, blacklist is implemented as []string. We should use a thread safe structure instead and make the required changes in the parts of the code that use it.