Read entire delete RID bitmap into memory for faster deleted RID detection

[greenlion]

Right now the deleted rid bitmap is read in in 8 byte chunks. That is a lot of fread() calls (even though the OS should buffer the reads). A table with 64 million rows only takes 8MB of space to store the bitmap entirely in ram, so it makes sense to read the whole thing in and just point ->bits to the proper offset in the buffer.