Clean up 4.0 compiler warnings

[CaveSpectre11]

Problem

Compiler warnings when building off the v4.0 tag

Root Cause

Several different issues:

init.cpp: In function ‘bool AppInit2()’:
init.cpp:1442:13: warning: unused variable ‘nStart’ [-Wunused-variable]
     int64_t nStart = GetTimeMillis();
             ^~~~~~

wallet/wallet.cpp: In member function ‘CAmount CWallet::GetCredit(const CTransaction&, const isminefilter&, bool) const’:
wallet/wallet.cpp:5649:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < tx.vout.size(); i++) {
                     ~~^~~~~~~~~~~~~~~~

leveldb/util/logging.cc: In function ‘bool leveldb::ConsumeDecimalNumber(leveldb::Slice*, uint64_t*)’:
leveldb/util/logging.cc:58:40: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
           (v == kMaxUint64/10 && delta > kMaxUint64%10)) {
                                  ~~~~~~^~~~~~~~~~~~~~~

qt/transactionrecord.cpp: In static member function ‘static QList<TransactionRecord> TransactionRecord::decomposeTransaction(const CWallet*, const CWalletTx&)’:
qt/transactionrecord.cpp:221:57: warning: ‘*’ in boolean context, suggest ‘&&’ instead [-Wint-in-bool-context]
         if (fAllFromMeDenom && fAllToMeDenom && nFromMe * nToMe) {
                                                 ~~~~~~~~^~~~~~~

In file included from qt/pivx/welcomecontentwidget.cpp:5:0:
./qt/pivx/welcomecontentwidget.h: In constructor ‘WelcomeContentWidget::WelcomeContentWidget (QWidget*)’:
./qt/pivx/welcomecontentwidget.h:44:18: warning: ‘WelcomeContentWidget::backButton’ will be initialized after  -Wreorder]
     QPushButton *backButton;
                  ^~~~~~~~~~
./qt/pivx/welcomecontentwidget.h:40:18: warning:   ‘QPushButton* 

WelcomeContentWidget::icConfirm1’ [-Wreorder]
     QPushButton *icConfirm1;
                  ^~~~~~~~~~
qt/pivx/welcomecontentwidget.cpp:15:1: warning:   when initialized here [-Wreorder]
 WelcomeContentWidget::WelcomeContentWidget(QWidget *parent) :
 ^~~~~~~~~~~~~~~~~~~~

Additional

One other I didn't touch.

qt/pivx/walletpassworddialog.cpp: In constructor ‘WalletPasswordDialog::WalletPasswordDialog(QWidget*)’:
qt/pivx/walletpassworddialog.cpp:85:9: warning: unused variable ‘posXX’ [-Wunused-variable]
     int posXX = ui->lineEditPassword->width() - 30;
         ^~~~~

Since I wasn't sure where exactly that dialog pops up to see if the posXX is actually needed and should have a btnWatch->move(); so i figured I would leave that to @furszy to take care of.

[Fuzzbawls]

leveldb changes will need to be proposed upstream to https://github.com/bitcoin-core/leveldb. Looks like existing discussion is at bitcoin-core/leveldb-subtree#25

[furszy]

@CaveSpectre11 just remove the logging.cc change, the rest is good to go.

[CaveSpectre11]

@CaveSpectre11 just remove the logging.cc change, the rest is good to go.

All set

[random-zebra]

utACK bccbec4

[Warrows]

ACK bccbec4