db_lmdb: do not cast const away
db_lmdb: remove block timestamp too when removing a block
blockchain: do not append "testnet" to the data directory

It is already there (unless overridden via command line).
blockchain_converter: only call data path function once
blockchain_converter: a bit more user friendly output
db_lmdb: factor the MDB_val setup code

It makes the code simpler, avoids possible copy/paste errors
(wrong sizeof, etc), and generally unclutters the calling code.
Merge pull request #15 from moneromooo-monero/blockchain

many const-correctness changes and code quality enhancements.  minor bugfix on block removal.