Cleanup in Preloader class

[hbdeshmukh]

Added missing include and more informational log messages.

[hbdeshmukh]

Hi @pateljm

Please review this PR. It has mostly simple changes. These changes can be potentially useful in cases when we decide not to preload the complete database (e.g. for experiments involving data sets larger than buffer pool), to check which relations have been preloaded to the buffer pool.

A useful feature (TODO) will be to specify which relations can be preloaded to the buffer pool.

[hbdeshmukh]

Addressed all the comments.