When closing, the PageManager now truncates unused pages at the end and reduces the file size
Improved naming consistency (renamed page_size/page_id to pagesize/pageid)
Minor refactoring in PageManager, no functional changes but improved error checks
Refactoring of env.h/env.cc: splitting in multiple files, improving code
Getting rid of internal_fwd_decl.h
Fixed bug in check_integrity when a return code was swallowed
Fixing several minor issues