[Performance] Reduce unnecessary copying of strings and objects

[marcomagdy]

Description of changes:

This patch attempts to improve performance by doing the following:

1. Remove an extra getenv call, which internally involve a bunch of string copying.
2. Avoid default constructing the invocation_request object only to overwrite it later with the actual request. Instead use a reference.
3. Avoid copying the request string when we can pass it by reference

Testing:

• make test
• make test-integ

Target (OCI, Managed Runtime, both):

Both

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[marcomagdy]

FWIW, I wrote the native the client.