Solr configuration tunables

Add some tunables to the solr plugin, to play with the HTTP client when using the external Solr server.

@Primer42 @dallasmarlow @maddalab
Not necessary, it is just code improvement. This is fine. The idea tho with ```optionals``` is to avoid get without a check.

```
if (SolrConfig.useEmbeddedServer) {
      Solr.getNewEmbeddedServer(SolrConfig.embeddedSolrHome)
     } else {
      SolrConfig.externalUrl.map(Solr.getNewRemoteServer(_))
     }
```

A second LGTM feel free to merge