Refactor Elasticsearch Log Exporter to use Synchronous HTTP methods

[MarkSeufert]

Is your feature request related to a problem?
The Elasticsearch exporter currently uses asynchronous http requests to communicate with the Elasticsearch instance, where the error and response of the request is received through callback methods. Since the Export() method needs wait for the result of the http request, it has to block until the response is received.

Describe the solution you'd like
Using synchronous http requests will simplify the design of the exporter, since there will no longer be a need for the callback methods. Instead, the http request call would look something like:
auto response = session->SendRequestSync();

Additional context
The synchronous functionality is being added in PR #448, which needs to be merged before this issue can be worked on.

cc - @alolita @xukaren

[lalitb]

This can be closed for now. Async http requests are the right approach, as this allows to Cancel/Abort the request as needed.