depends: add a fallback path in case package sources go missing

If a source url fails to download, try again at
$FALLBACK_DOWNLOAD_PATH/file.name, where FALLBACK_DOWNLOAD_PATH can be
overridden by the user.
Move UpdateTime to pow
Move CBlockIndex::GetBlockWork() to pow::GetProofIncrement(nBits)
replace ComputeMinWork with CheckMinWork
Bypass reloading blocks from disk
depends: add sensible download timeout/retry values
Merge pull request #4752

df7565d depends: add sensible download timeout/retry values (Cory Fields)
c897b1e depends: add a fallback path in case package sources go missing (Cory Fields)
Revert "Add a getutxos command to the p2p protocol. It allows querying of the UTXO set"

This reverts commit da2ec100f3681176f60dec6dc675fc64147ade3a.
Merge pull request #4764

92bb6f2 Bypass reloading blocks from disk (Pieter Wuille)
Merge pull request #4377

654871d replace ComputeMinWork with CheckMinWork (jtimon)
b343c1a Move CBlockIndex::GetBlockWork() to pow::GetProofIncrement(nBits) (jtimon)
c2c02f3 Move UpdateTime to pow (jtimon)
Fix build with DEBUG_LOCKORDER

Fixes #4771
fix comments ExtractAddress() -> ExtractDestination()
qt/splashscreen: #include version.h

Needed to build breakage reported by Arnavion on IRC:
qt/splashscreen.cpp: In constructor 'SplashScreen::SplashScreen(const QPixmap&, Qt::WindowFlags, bool)':
qt/splashscreen.cpp:33:98: error: 'FormatFullVersion' was not declared in this scope
Add travis build:passing/failing indicator.
Merge pull request #4766

5a0823a Add travis build:passing/failing indicator. (randy-waterhouse)
Merge pull request #4772

87d9819 fix comments ExtractAddress() -> ExtractDestination() (jtimon)
gitian: descriptors overhaul

Descriptors now make use of the dependencies builder, so results are cached.
A very new version (>= 611292cd) of Gitian should be used in order to take
advantage of caching.
quick docs update
gitian: enable glibc-compat for linux builds and reduce symbol visibility

Precise's bfd linker doesn't honor "-Wl,--exclude-libs,ALL" properly. It
seemingly just ignores the option. Gold works as intended.

Installing binutils-gold switches the default linker, so that's all it takes to
fix.
depends: disable ssl2 in openssl build

Ubuntu disables it by default for security reasons. If it's enabled, its
symbols can leak into our builds, causing release binaries to fail at runtime
on some distros.