[feat] use couch _all_docs for get_docs() and get_all_docs()

The previous solution would make use of concurrent get's to couch
backend in a pool of threads to implement the get_docs() and
get_all_docs() CouchDatabase backend methods.

This commit replaces those by a simpler implementation use the
`_all_docs` couchdb view api. It passes all needed IDs to the view and
r etrieves all documents with content in the same request.

A comparison between both implementations shows an improvement of at
least 15 times for large number of documents. The table below shows the
time for different implementations of get_all_docs() for different
number of documents and threads versus _all_docs implementation:

  +-------+-----------------+------------------+-------------+
  |       |     threads     |    _all_docs     | improvement |
  +-------+-----------------+------------------+-------------+
  |    10 | 0.0728030204773 | 0.00782012939453 |      9.3    |
  |   100 | 0.609349966049  | 0.0377721786499  |     16.1    |
  |  1000 | 5.86522197723   | 0.370730876923   |     15.8    |
  | 10000 | 66.1713931561   | 3.61764383316    |     18.3    |
  +-------+-----------------+------------------+-------------+

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