Check against the right aux block version number

Previous commit 9e9babb was based on a wrong assumption, actual
CBlockHeader->nVersion for auxpow blocks is
CBlockHeader::CURRENT_VERSION | (AUXPOW_CHAIN_ID
   * BLOCK_VERSION_CHAIN_START)

and not BLOCK_VERSION_AUXPOW
Apply predetermined block for testnet fork

PR #589 caused a condition for testnet difficulty to not match at
a couple of cases pre block 145k leading to issues with -reindex.

This makes the testnet fork hard at block 157500, while retaining
every case before that block.
Added missing step of deleting cursor in case of an error reading blocks from disk.
Pointer for GetStats() is now scoped rather than manually deleted.