couch_index: add background indexing facility

This change add the possibility to trigger a view indexation in
background. The indexation can only work in background if at least one
process acquired it using the `couch_index_server:acquire_index/3`
function. If all the process that acquired it are down or released it
using `couch_index_server:release_indexer/3` then the background task is
stopped.

By default the background indexation will happen every 1s or when 200
docs has been saved in the database. These parameters can be changed
using the options `threshold` and `refresh_interval` in the couch_index
section.