MB-7904: Use byte size instead of number of nodes

The R-tree now uses the byte size of the nodes to determine
how many children a node can hold. Previously it was determined
by a minimum and a maximum number of nodes.

This step was taken to make the R-tree work more like the
B-tree code.

Change-Id: Ifc4f08110a9ebe11385e0bbb63a53b34de732691
Reviewed-on: http://review.couchbase.org/25138
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>