refactor instruments and add appium-internal flake support

This commit constitutes a pretty big refactor of the instruments
process handler. It also introduces a server flag, '--backend-retries',
with a default of '3', which specifies how many times Appium will retry
launching instruments before giving up and saying that it crashed or
timed out. This works in concert with launchTimeout, of course.

I think this refactor also fixes some longstanding callback reasoning
errors and maybe some other bugs.

It might also introduce some bugs of its own though :-)

docs/server-args.md

@@ -16,6 +16,7 @@ All flags are optional, but some are required in conjunction with certain others
 |`-p`, `--port`|4723|port to listen on|`--port 4723`|
 |`-dp`, `--device-port`|4724|(Android-only) port to connect to device on|`--device-port 4724`|
 |`-k`, `--keep-artifacts`|false|(IOS-only) Keep Instruments trace directories||
+|`-r`, `--backend-retries`|3|(iOS-only) How many times to retry launching Instruments before saying it crashed or timed out|`--backend-retries 3`|
 |`--session-override`|false|Enables session override (clobbering)||
 |`--full-reset`|false|(Android-only) Reset app state by uninstalling app instead of clearing app data||
 |`--no-reset`|false|Don't reset app state between sessions (IOS: don't delete app plist files; Android: don't uninstall app before new session)||

lib/appium.js

@@ -546,6 +546,7 @@ Appium.prototype.initDevice = function() {
       , isSafariLauncherApp: this.isSafariLauncherApp
       , desiredCapabilities: this.desiredCapabilities
       , logNoColors: this.args.logNoColors
+      , flakeyRetries: this.args.backendRetries
     };
     if (this.desiredCapabilities.safari || this.desiredCapabilities.iwebview) {
       this.device = new Safari(iosOpts);