Per-peer block download tracking and stalled download detection.

Keep track of which block is being requested (and to be requested) from
each peer, and limit the number of blocks in-flight per peer. In addition,
detect stalled downloads, and disconnect if they persist for too long.

This means blocks are never requested twice, and should eliminate duplicate
downloads during synchronization.
convert tabs to whitespace in bitrpc.py

This is a minor change to bitrpc.py to conform with the formatting of
the other python source code in the repo.
Fix typo in doc/build-unix.md regarding building without UPnP
[Qt] show number of in/out connections in debug console
MainNet is the special case for the GUI
Minor code cleanup: remove indentation

This indentation should have been stripped out when AreInputsStandard was made a top-level function instead of a CTransaction method.
Merge pull request #3823

89ec3a2 MainNet is the special case for the GUI (jtimon)
Merge pull request #3817

e935293 Minor code cleanup: remove indentation (Mark Friedenbach)
Merge pull request #3773

947e23d Fix typo in doc/build-unix.md regarding building without UPnP (Rune K. Svendsen)
Merge pull request #3751

b881100 convert tabs to whitespace in bitrpc.py (Bryan Bishop)
Merge pull request #3685

8e29623 [Qt] show number of in/out connections in debug console (Philip Kaufmann)
qt: Adjust BLOCK_CHAIN_SIZE to 20GB

This increases the space requirement for selecting a data directory in
the UI.
As suggested by @bardiharborow, fixes #3830.
add --enable-debug for configure
Merge pull request #3833

f5f157b add --enable-debug for configure (daniel)
Merge pull request #3514

f59d8f0 Per-peer block download tracking and stalled download detection. (Pieter Wuille)
doc/build-unix.md  use github flavored md for table
Transifex pull pre-0.9.0rc3/final
Merge pull request #3835

064a690 Transifex pull pre-0.9.0rc3/final (Wladimir J. van der Laan)
Merge pull request #3829

a75cacd doc/build-unix.md  use github flavored md for table (Torstein Husebø)