Give replica set secondaries votes in concurrency suites

Presently, by default we give zero votes to secondary members in concurrency suites' ReplicaSetFixture . As a result, lastCommitted lag is not reliable because non-voting members are not considered in its calculation. This means that consumers of lastCommitted lag, such as Flow Control, will not have much testing coverage in most concurrency suites.

In order to prevent a new primary from being elected, it is sufficient to give secondary members zero priority. IIUC, giving secondaries zero votes just prevents stepdowns of the current primary.

We should consider removing the restriction on secondary votes. If it is absolutely necessary to avoid stepdowns in certain cases, we should make sure that these cases are handled individually.