Contextualize the BufferCache's pin, upin, and write operations

The buffer cache's pin, unpin, and write operations are not exposed to the context of the overall on going operation. To some extent, the pin operation have some context (e.g., not incrementing stats). Changing the buffer cache interface every time we want to incorporate contextual information is not sustainable. Instead, a user of the buffer cache should provide a context object that can hold all the necessary information of a given buffer cache operation (e.g., incrementing stats or processing the header when reading a page in a pin operation).