Add an adaptor to es_objects and template function to reduce code

[oxarbitrage]

The following pull request is an attempt to reduce the amount of code in the es_objects plugin and make it easier to add new objects. It uses an adaptor approach very similar to the one used in elasticsearch plugin for account history, this time for objects.

I think more can be done, specifically there is duplicated code in index_database function that i tried to reduce however i was not able to make it due to casting, types, etc.

I opted on submitting this changes as i think they go in the right direction, as i don't know exactly when i will have the time to work on this again i preferred to submit what i have now and don't let the code lost.

This is a refactor, users can continue using this plugin the same way they were doing before without changes in the options however i am considering it will be better to add the objects the user want to save in a vector instead of individual options for each as it is now.

In initial testings i was able to sync without any error up to 2016-05-10T02:10:30 with everything turned on(all objects) with command:

./programs/witness_node/witness_node --plugins "es_objects" --es-objects-proposals "true" --es-objects-accounts "true" --es-objects-assets "true" --es-objects-balances "true" --es-objects-limit-orders "true" --es-objects-asset-bitasset "true"

[pmconrad]

Looks like an improvement.
Haven't tested it myself, but it's an experimental plugin so if it works for you it should be ok.

[oxarbitrage]

thank you @pmconrad