[Qt] New status bar Unit Display Control and related changes.

- New status bar control shows the current Unit of Display.
  When clicked (left,or right button) it shows a context menu
  that allows the user to switch the current Unit of Display (BTC, mBTC, uBTC)
- Recent Requests and Transaction Table headers are now updated when
  unit of display is changed, because their "Amount" column now displays the
  current unit of display.
- Takes care of issue #3970 Units in transaction export csv file.
- Small refactors for reusability.
- Demo Video https://www.youtube.com/watch?v=wwcr0Yh68go&list=UUG3jF2hgofmLWP0tRPisQAQ
- changes after Diapolo's feedback. Have not been able to build after last pool, issues with boost on MacOSX, will test on Ubuntu these changes.
- removed return statement on switch
- renamed onDisplayUnitsChanged(int) to updateDisplayUnit(int)
- now getAmountColumnTitle(int unit) takes a simple unit parameter. moved to BitcoinUnits.
Use async name resolving to improve net thread responsiveness

In the LookupIntern(), things changed are:

  1. Call getaddrinfo_a() instead of getaddrinfo() if available, the former is a sync version of the latter;
  2. Try using inet_pton()/inet_addr() to convert the input text to a network addr structure at first, if success the extra name resolving thread inside getaddrinfo_a() could be avoided;
  3.  An interruption point added in the waiting loop for return from getaddrinfo_a(), which completes the improve for thread responsiveness.

A easy way to see the effect is to kick off a 'bitcoind stop' immediately after 'bitcoind -daemon', before the change it would take several, or even tens of, minutes on a bad network situation to wait for the running bitcoind to exit, now it costs only seconds.

Signed-off-by: Huang Le <4tarhl@gmail.com>
Check signatures before respend relay

Check that all inputs are completely valid before actually
relaying a double-spend.
Fix formatting in release-process.md
Update Gitian Build guide to include OSX
Remove signal DoubleSpendDetected, use function

Also removes the need for forward reference to RelayableRespend.
CBlockIndex()::SetNull() method to avoid code repetition
Merge pull request #4454

9d5ad71 Fix formatting in release-process.md (Michael Ford)
Merge pull request #4455

3f7a61f Update Gitian Build guide to include OSX (Michael Ford)
Merge pull request #4302

8969828 [Qt] New status bar Unit Display Control and related changes. (gubatron)
Use fee/priority estimates in wallet CreateTransaction

The wallet now uses the mempool fee estimator with a new
command-line option: -txconfirmtarget (default: 1) instead
of using hard-coded fees or priorities.

A new bitcoind that hasn't seen enough transactions to estimate
will fall back to the old hard-coded minimum priority or
transaction fee.

-paytxfee option overrides -txconfirmtarget.

Relaying and mining code isn't changed.

For Qt, the coin control dialog now uses priority estimates to
label transaction priority (instead of hard-coded constants);
unspent outputs were consistently labeled with a much higher
priority than is justified by the free transactions actually
being accepted into blocks.

I did not implement any GUI for setting -txconfirmtarget; I would
suggest getting rid of the "Pay transaction fee" GUI and replace
it with either "target number of confirmations" or maybe
a "faster confirmation <--> lower fee" slider or select box.
Sanity checks for estimates

Require at least 11 samples before giving fee/priority estimates.

And have wallet-created transactions go throught the fee-sanity-check
code path.
Move fee policy out of core
Merge branch 'smartfee_wallet'
Show nodeid instead of addresses (for anonymity) unless otherwise requested.
Merge pull request #4450

0da6b3f Remove signal DoubleSpendDetected, use function (Tom Harding)
88dd359 Check signatures before respend relay (Tom Harding)
Merge pull request #3764

2e36866 Show nodeid instead of addresses (for anonymity) unless otherwise requested. (R E Broadley)
Add libtool generated files to .gitignore
Merge pull request #4421

caf6150 Use async name resolving to improve net thread responsiveness (Huang Le)
Use pong receive time rather than processing time