Support reusable buffers

[kriszyp]

napi_*_buffer_*, and napi_*_named_property methods are very expensive, we would like to provide a way to avoid calling these methods for the majority of CRUD operations. Specifically, we would like to be able to reuse buffers for writing keys and values, and when a previously used buffer contains a key or value, the put, delete, get, and addEntry functions can utilize the already determined address of the buffer rather than having to calculate it again with these methods. (We have discussed a number of different ways of communicating this through the native layer, but the general goal of this ticket is ensuring some mechanism can be used to avoid these calls when possible).