Backport: set vreplication net read and net write timeout session vars to high values

[pawandubey]

Backport of vitessio@6953390
Redo of #129 onto the newest release.
Part of https://github.com/Shopify/vitess-project/issues/623

Helps with failure of schema changes using vitess strategy due to lower than desired timeout values.

⚠️ Does not contain bits from the original PR that reference features that were introduced in releases later than 15. (e.g. change in tablestreamer.go introduced in vitessio@04ad6ec, and in rpc_vreplication_test.go which was introduced in vitessio@20e4768).

[pawandubey]

The previous attempt also passed these flags to vtcombo which is used in vttestserver so would be useful in dev and tests, but given that upstream PR does not add it, I am wary of adding to it. Also unsure if the schema migration issue will crop up in dev anyway.

[shivnagarajan]

makes sense to leave it out.

[pawandubey]

Two checks are failing for non-code reasons:

1. Unit Test (mariadb103) / test (pull_request) is failing because one of the repository links no longer exists. The rackspace mirror now only hosts mariadb 10.4 and later, and not 10.3: https://mirror.rackspace.com/mariadb/repo/. I think it's okay to ignore this as we don't use mariadb.
2. Static Code Checks, Etc. / Static Code Checks Etc (pull_request) is failing because an API token for the static code scanning platform FOSSA was changed and this version of the code does not have it. I think this is okay to ignore also?

[shivnagarajan]

makes sense to leave it out.

[shanth96]

Do we also want to port the tests?

[pawandubey]

Do we also want to port the tests?

@shanth96 the tests were added to a file that does not yet exist in v15. I tried to note that in the PR description. I did not want to introduce the file here since the tests are rather simplistic? Happy to be convinced otherwise.

[brendar]

LGTM!