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 #156 Add a parameter for the nonleaf partial eviction strategy
fixes #153 Add parameter for the write on leaf flush strategy. If
enabled, we clone and write out a leaf node after it's flushed to.