potential cleanups possible before 0.8 RC #2230

Some changes/improvements to code that is new for 0.8 are still possible before doing RCs. These include:

• Dropping support for the pre-checksum undo file format (#2216)
• Changing the directory naming scheme. From a discussion on IRC it was apparent that for example the name coins/ was confusing (people thought it was related to the wallet). One suggestion:
  • blocks/undo files remain in blocks/*
  • blktree/ becomes blocks/index/* (so it is intuitively clear it's associated with the blocks directory)
  • coins/ becomes chaindata/
  • optionally: put everything inside a subdirectory blockchain/, and perhaps make it configurable with -blockchaindir=, as it is now independent from the wallet anyway.
• Adding an automatically generated README in the data directory/directories to explain what the different files/directories contain (suggested by andytoshi on IRC).
• Calling an ftruncate() (or windows equivalent) on the block files when abandoning them would remove the permanent storage waste caused by pre-allocated padding being left over.

I tagged #2216 for 0.8

Changing directory naming scheme: yes, I think that should be done now, it will save lots of time explaining in the future. I don't care what the naming scheme is, just pick one and make it so.

Auto-generated README: I say no, we can do that in a future release. ftruncate() : I say no, disk space is cheap. Time would be better spent testing a more important feature (if we care about disk space, then pruning would be that more important feature; I don't care much about disk space, though).

#2231 was merged.