Support sequence id type on MongoDB

Like for any SQL db having compact document id improves index IO and memory footprint.
We should support more compact ecm:id type in the form of a sequence computed by the database.

For performance reason, to avoid a round-trip whenever we need a new sequence id, we should provide a way for sequence allocation to be done by blocks.

–
Use of a MongoDB sequence for ids, and definition of the block size, can be activated in the repository's configuration using:

    <repository name="default" label="label.default.repository">
      <server>${nuxeo.mongodb.server}</server>
      <dbname>${nuxeo.mongodb.dbname}</dbname>
      ...
      <idType>sequence</idType>
      <sequenceBlockSize>100</sequenceBlockSize>
      ...
    </repository>