Add new function to Filehandle

From the reader’s description:

In order for you to gather performance data about your linear hashing
index, we would like you to tweak the FileHandle class to maintain
counters for each I/O-related operation of its FileHandle class. We
will also add one more method to get a read-out of these counter
values. When initialized, all counters should be set to 0. Inside each
method of FileHandle, you will then need to increase the corresponding
counter by 1 every time you successfully execute the associated method.
For instance, when readPage() is executed, readPageCounter should be
increased by 1.

Again, since a Linear Hashing index FileHandle will have pointers to
multiple PF FileHandles, you will have some way to link multiple PF
file handles in order to perform index operations. At the index logical
API level, you will need to return cumulative counter values for those
FileHandles (i.e., you will add them up) when you are asked to produce
index-level operation counts.