(WIP) Add option to import json dump file (#291)

[hbruch]

This is a first attempt to provide a json dump import (see #291). Importing a json dump on my machine was about 10x faster than importing from nominatim.

However, there are some points I'd like to receive feedback on before doing a PR for merge:

• currently, no sanity checking is done on import, neither of the action line, nor the source line, should it?
• the JsonDumper does not export the PhotonDoc.uid. It should either be exported or the PhotonDoc should be restored from json so it can be recalculated from source
• JsonDumpConnector does not convert a source line into a PhotonDoc but bypasses the public Importer interface to add the source string. This is ugly, but avoids (unnecessary?) parsing and formatting of the source (but would require uid handling, see above)
• the elasticsearcg.Importer uses BulkRequestBuilder. Using BulkProcessor would provide concurrent updates, as well as retries with exponential backup in case of failures (this should be another PR, I think)

[karussell]

Here we should throw a RuntimeException to avoid usage of this connector.

[karussell]

shouldn't it be provided on construction and then importer and reader could be final?

[karussell]

currently, no sanity checking is done on import, neither of the action line, nor the source line, should it?

Sanity of which form like empty content?

It should either be exported or the PhotonDoc should be restored from json so it can be recalculated from source

Which option do you prefer?

This is ugly, but avoids (unnecessary?) parsing and formatting of the source (but would require uid handling, see above)

Can't judge about it. Both methods seems ok to me.

this should be another PR, I think

Yes, speed is not important for the first implementation IMO

[karussell]

Have included some improvements of this in #438