... potentially saving a lock/unlock on the critical vcl_mtx

Both callers of vcl_get(), VPI_vcl_select() and VCL_Refresh(),
potentially have to unref another vcl first.

For VCL_Refresh(), this happens when the active vcl changes and the
previously active vcl is cached in (struct worker).vcl, which can be
assumed to always be the case on a busy system.

For VPI_vcl_select(), this happens when switching vcls at
esi_level > 0.

To summarize, this patch will primarily reduce contention on the
vcl_mtx after a 'vcl.use' CLI command on a busy system and for
'return(vcl(...))' with ESI.