fix for broken template mapping json

[Munawwar]

Seems like the mapping part of json file needs to be wrapped into a "type" (which can be any name). This is on elasticsearch 6. I am not sure if older elastic used to accept without it.

e.g.

"mapping": {
  "asdasd": {
     "properties": { .... }
  }
}

[coveralls]

Coverage decreased (-0.7%) to 86.634% when pulling 26f4172 on Carriyo:fix-mapping-template into 64c0bdd on vanthome:master.

[Munawwar]

Ok. I've changed a couple of things

1. There had to be two template.json files - one for elasticsearch version 7 & above, and another one for elasticsearch version 6 & below

2. Which means there has to be a way to know which ES version is being used. I introduced elasticsearchVersion field. As I think making a request for a not-so-common case (i.e. ensureMappingTemplate = true + mappingTemplate = undefined combination) is a bit of an overkill.. what do you think?

3. However for tests, to test without specifying the template json file like it is now, I had to make a call to ES to find out the version.

4. There is a bug in how log writing starts before ensuring template mapping succeeds. So fixed that as well.

5. Another blocking bug I found on my setup is that the health check code fails with error 408 when you have more than one node in your elasticsearch cluster. I guess because the health check is hard-coded to '1'. It should rather be '>=1'.

[vanthome]

can you please rebase it?

[Munawwar]

@vanthome Done.

[teyou]

Pls help update index.d.ts as well as there are more options in ElasticsearchTransportOptions.

related : #134

[Munawwar]

Added it now