Capture "number of documents modified/deleted" for operations on document stores

## Problem description

This is an offshoot of https://github.com/elastic/apm/issues/112. That issue is scoped to SQL databases, which typically report the number of "rows affected" in response to DML statements (INSERT, UPDATE, DELETE, ...).

The solution for that specific issue (report "rows_affected") does not make sense for document stores, such as Elasticsearch and MongoDB. At least those two separate the number of modifications/updates vs. deletions, and they do not operate on rows but rather documents, thus "rows_affected" is not sensible here.

## Solution proposal

Introduce the following span context:

- `context.db.docs_modified` (created or updated)
- `context.db.docs_deleted`

How they are stored in Elasticsearch is TBD.