I thought that the merge was implicated for making nodes too big, but now I'm convinced it's not.

When the brt tries to merge two nodes (maybe_merge_pinned_leaf_nodes) it has two cases
 1) The nodes combined are more than 3/4 a node, in which case it balances the nodes.
 2) Otherwise it merges them.  Hence merged nodes are always smaller than a node.

So I'm considering #1802 and #1811 to be fully resolved.

Refs #1802.  Closes #1811


git-svn-id: file:///svn/toku/tokudb@12836 c7de825b-a66e-492c-adef-691d508d4ae1