In buffer manager, `numPages` is not changing when the file has more pages.

[Gates Wang]

Please see CLOCK.c, FIFO.c and LRU.c.

When ensureCapacity() is used to extend the num of pages in that page file, the numPages in the buffer manager remains the same.

[Gates Wang]

先从master创建分支 issue_3 然后修改之后做Pull Request

[Gates Wang]

It's my fault. The numPages in BufferManager is the num of pages that a manager holds, not the num of pages that the file contains.