MB-100: Default fill_min/fill_max to `nil`

fill_min/fill_max in the #vtree{} record are `nil` by default
and will set by the corresponding API (Couchbase or Apache CouchDB).

Change-Id: I11814f6982f5792c3f07f73a4770811e6cc294af
Reviewed-on: http://review.couchbase.org/25135
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
MB-7013: Update for recent Apache CouchDB 1.3.x changes

COUCHDB-1334 [1] was reverted in a8aff9f91790b0dae347b10b7c11b21f34faa0b2 [2],
hence GeoCouch needed to be adapted as well.

[1] https://issues.apache.org/jira/browse/COUCHDB-1334
[2] https://github.com/apache/couchdb/commit/a8aff9f91790b0dae347b10b7c11b21f34faa0b2

Change-Id: I5f31e390accbc27ba9b409b465b8e7d32e26d906
Reviewed-on: http://review.couchbase.org/25136
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
MB-7904: Write body and geometry early

Write the body and the geometry of a KV-node early. This is needed
to make the size calculations easier, that are needed to determine
how many children a node will contain.

This means that the body and the geometry of nodes that will be
inserted are written right at the beginning. All calculations
about splitting etc. will happen afterwards.

Change-Id: Iff48a19ee6903db24270adf9273bd5d7d8a85a08
Reviewed-on: http://review.couchbase.org/25137
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>