Add communication graph to database

- Add communication graph built up from email exchanges
  to the database

Signed-off-by: Mitchell Joblin <mitchell.joblin.ext@siemens.com>
Refactor mechanism to store graph data into the datbase

- separate the graph indexing manipulation from the database
  writing functionality
- remove redundant cluster user index mapping, we already do
  that to get the edge lists

Signed-off-by: Mitchell Joblin <mitchell.joblin.ext@siemens.com>