fixes #127 Add a class for scoped mallocs, which are satisfied from a
thread-local buffer when possible.
fixes #145 Destroy resources outside of the pair's write lock during
partial eviction
fixes #146 Add context class and track the current writer/context in the
frwlock
fixes #153 Parameterize the partial eviction strategy for nonleaf nodes
and add a parameter for whether leaf nodes get written out on flush.