Add the Server Name Indication (SNI) field to healthcheck requests, if
present in the target config.
SNI handling is already present for proxying resources, but this has
not been implemented for healthcheck requests yet until this commit.
In order to facilitate this, this commit moves the client instantiation
to when the healthcheck is performed, rather than as a part of the
monitor init.
Former-commit-id: 8a45f6fd63
Owen