After DeepCopy improvements, init context time takes roughly 39s,
and more than 20% of cpu time is spent on the VirtualServicesForGateway
function:
https://github.com/istio/istio/blob/1.14.4/pilot/pkg/model/push_context.go#L863-L875

This function is called for every sidecar's egress host, for calculating
the virtual services that are imported by the egress host. We have more
than 10k sidecars and suppose each sidecar has 10 egress hosts, this
function is called 100k times. What makes it worse is that all our
virtual services are public (exportTo: *), so VirtualServicesForGateway
creates and copies all virtual services (roughly also more than 10k)
each time. This "make slice" and "slice copy" are expansive on such
magnitude.

This CL gets rid of such copy, instead of passing in the copied and
merged version of the virtual services, just pass the
virtualServiceIndex into the select function directly. This improves
the init context time to roughly 25s.

Change-Id: I48015e750a1019f12dfc35b0ca42b72fddfa87ba
Reviewed-on: https://gerrit.musta.ch/c/public/istio/+/3745
Reviewed-by: Jungho Ahn <jungho.ahn@airbnb.com>
Reviewed-by: Weibo He <weibo.he@airbnb.com>