Fixes #348 In stub mode even the last invocation is irrelevant

[bric3]

Hi there,

After re-reading #348, I think the invocation reservoir, doesn't have to keep the last invocation.

This PR changes the semantics of the invocation reservoir, so no invocations are stored.

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[davidburstromspotify]

Hi! Will this PR be worked on?

[eduardbosch-jt]

Hi!
What is the status of this PR?
I think that it makes sense to not store any invocation when using configuring a mock with stubOnly.

I'm actually having come leaks due to this.

I'm trying to remove them with clearInvocations, but anyway, it looks like stubOnly should not store any invocation reference.