DB Store Abstractions

[janos]

This PR introduces a small package with database abstractions that may be used for swarm storage layer. Closes: #989.

This PR is not required to be merged into the master branch until there is an implementation that depends on it.

This PR is open for reviews and suggestions on design and new features. It is founded on requirements described in https://hackmd.io/ffBjSu8RTyKikvRO7bYrzA#.

Future generalization: #1001.

[zelig]

need to add a bool field to control if Index encode/decode should actually fetch data from mock globalstore

[acud]

Great work on this @janos

There's a few comments in the body of the PR. one thing that I particularly don't really understand is that UseMockStore bool pointer. does this somehow relate to the current (on master) db layout with the mockStore component injection (which is in fact just a store). In general I would like to avoid using field names that include mock(s) in production code.

thanks <3 🙏

[acud]

1. @zelig why?
   1.1. Address and Data are nullable
2. I still fail to understand what the field name signifies

[janos]

I still fail to understand what the field name signifies

@justelad This field was left by my mistake after an attempt to have a mock store integration in shed. We decided that it may be better to leave this integration for localstore or a higher level package.

I will remove this field.

[janos]

1. @zelig why?
   1.1. Address and Data are nullable
2. I still fail to understand what the field name signifies

@justelad I missed this comment while writing responses here is a brief explanation #993 (comment).

[janos]

Thank you @justelad for a review and @zelig and @nonsense for comments. I have addressed your suggestions. Please check if they are ok, especially the metrics part.

[zelig]

address @justelad and @nonsense then we can submit to upstream

[janos]

Submitted upstream ethereum/go-ethereum#18183.