Add mutex to write operations on mongo db

[pablomendezroyo]

Add mutex to write operations on mongo db, I recreated the issue where having multiple brains sending requests to the listener multiple bsons were created for the same pubkey tag and network

[Marketen]

adding mutex is a good safety measure, but we will probably never have two brains sending requests for the same validator, right?

[pablomendezroyo]

You are right, will leave the issue as opened anyway