Delete existing will message on client connect

[robertsLando]

#331

[robertsLando]

I also would suggest to keep just 1 retained message in db foreach client. Actually mondodb allows multiple will messages to be stored for the same clientid, In putWill function there should be a findAndUpdate with upsert true

[mcollina]

Would you mind to add a unit test for this feature?

[robertsLando]

@mcollina done

[mcollina]

lgtm