Add single connection performance scenarios

[glbrntt]

Motivation:

Running the unconstrained performance benchmark on a development
machines yields results with a large amount of variance (I have seen
over 20% between runs in some cases) since all cores become fully
loaded. As such it's much harder to determine whether a change was
actually beneficial.

Modifications:

• Add a single connection unary scenario, a copy of the unconstrained
  unary scenario but uses a single client such that only two threads are
  used: one for the server and one for the client.

Result:

• Results with less variance between runs.

[glbrntt]

cc @PeterAdams-A