[Upstream] [DB] Bitcoin 0.12-0.14 dbwrapper improvements

[lyricidal]

f4e6c52 from bitcoin/bitcoin#12659

The leveldb::Status class logs the filename of corrupted files, which might be useful when looking at error reports from usres. In theory this is already logged via the LogPrintf() statement in HandleError(), but that may not always be close to where the final error message is logged, e.g. see #11355 (comment) where the log trace provided by the user does not contain that information (and other user comments in the same issue).

This also adds a log message instructing the user to run the process with -debug=leveldb, which provides much more verbose error messages about LevelDB internals. This may not really help much, but improving the error messages here can't hurt.

The rest of the commits:

This backports a series of updates and cleanups to the LevelDB wrapper from:

• Change hardcoded character constants to descriptive named constants for db keys bitcoin/bitcoin#5707
• Obfuscate chainstate bitcoin/bitcoin#6650 [*]
• Unobfuscate chainstate data in CCoinsViewDB::GetStats bitcoin/bitcoin#6777 [*]
• Fix chainstate serialized_size computation bitcoin/bitcoin#6865
• [cleanup] leveldbwrapper becomes more generic 'dbwrapper' module bitcoin/bitcoin#6873
• Minor changes to dbwrapper to simplify support for other databases bitcoin/bitcoin#7927 [*]
• [Trivial] Do not shadow members in dbwrapper bitcoin/bitcoin#8467
• rpc: make gettxoutsettinfo run lock-free bitcoin/bitcoin#6290
  f4e6c52 from
• Refactor: Remove using namespace from bench/ & test/ sources bitcoin/bitcoin#9281

PIVX-specific edits were required to keep the sporks and zerocoin databases in line.

[*] NOTE: excluding the obfuscation of databases by xoring data, as we might not want this feature (e.g. as zcash/zcash#2598). Otherwise it can be discussed, and added, with a separate PR.

from PIVX-Project/PIVX#1629