Add optional transaction index to databases

By specifying -txindex when initializing the database, a txid-to-diskpos
index is maintained in the blktree database. This database is used to
help answering getrawtransaction() RPC queries, when enabled.

Changing the -txindex value requires a -reindex; the client will abort
at startup if the database and the specified -txindex mismatch.
Replace leveldb/ with vanilla 1.7.0
Remove Snappy support
Native Windows LevelDB port

Import native Windows LevelDB port by Hiram Chirino.

Extracted from from https://github.com/chirino/leveldb.git using
git diff dd0d562..aea83b7
Pre-Vista leveldb::port::InitOnce implementation
Mingw support for Windows LevelDB port
Port leveldb to MinGW32

Several changes to make the native windows leveldb code compile
with mingw32 and run on 32-bit Windows:
* Remove -std=c++0x dependency (modified code to use NULL instead of
  nullptr)
* Link with -lshlwapi
* Only #define snprintf/etc if compiling with Visual Studio
* Do not link against DbgHelp.lib (wrote a CreateDir instead of using
  DbgHelp's MakeSureDirectoryPathExists
* Define WINVER=0x0500 so MinGW32 can use the 64-bit-filesystem Windows
  api calls
* Define __USE_MINGW_ANSI_STDIO=1 to use MinGW's printf (which supports
  %ll)

I also cleaned up makefile.mingw, assuming that dependencies would be in
the standard /usr/local/{include,lib} by default but allowing overriding
with make DEPSDIR=... etc
added utility to dump leveldb files
Fix corruption bug found and analyzed by dhruba@gmail.com

https://groups.google.com/d/msg/leveldb/Kc9JxuIUu5A/9P0N9RL4ar8J
Bitcoin-Qt: cleanup / optimise addressbookpage

- don't show QR Code context menu, when USE_QRCODE=1 was not specified
  when compiling the client
- re-work on_showQRCode_clicked() for better readability and remove an
  unneeded duplicate check
- re-work on_signMessage_clicked() and on_verifyMessage_clicked() to match
  foreach in on_showQRCode_clicked(), which seems more robust / cleaner
- re-order context menu stuff to match real context menu layout
- add comments for all private slots in the class
Merge pull request #2165 from Diapolo/Qt_addrbook_cleanup

Bitcoin-Qt: cleanup / optimise addressbookpage
Merge pull request #2207 from gavinandresen/leveldb19

Leveldb19
Merge pull request #2168 from sipa/txindex

Add optional transaction index to databases
Added configure script.