tree 4f206185bd8c0ae02a278ff98f36fe83a0ce0777
parent f4a58847d3de70b359e57b57b59f4825afdb58c6
author Jason Evans <jasone@canonware.com> 1459159595 -0700
committer Jason Evans <jasone@canonware.com> 1464982053 -0700

Add element acquire/release capabilities to rtree.

This makes it possible to acquire short-term "ownership" of rtree
elements so that it is possible to read an extent pointer *and* read the
extent's contents with a guarantee that the element will not be modified
until the ownership is released.  This is intended as a mechanism for
resolving rtree read/write races rather than as a way to lock extents.