Combine CCoinsViewCache's HaveCoins and const GetCoins into AccessCoins.

The efficient version of CCoinsViewCache::GetCoins only works for known-to-exist
cache entries, requiring a separate HaveCoins call beforehand. This is
inefficient as both perform a hashtable lookup.

Replace the non-mutable GetCoins with AccessCoins, which returns a potentially-NULL
pointer. This also decreases the overloading of GetCoins.

Also replace some copying (inefficient) GetCoins calls with equivalent AccessCoins,
decreasing the copying.
qt: Remove thousands separators after decimal point

Revert thousands separators after decimal point, as introduced in #4167.
netbase: Use .data() instead of .c_str() on binary string

`.c_str()` is only guaranteed to return the data up to the first NUL
character.
qt: fix tablet crash. closes #4854.

This backports the relevant parts of:
https://codereview.qt-project.org/#/c/82689/
Remove some unnecessary c_strs() in logging and the GUI

Includes `core: remove unneeded c_str() / Qt: replace c_str() with Qt code`
by P. Kaufmann.
Merge pull request #4868

2c2cc5d Remove some unnecessary c_strs() in logging and the GUI (Philip Kaufmann)
f7d0a86 netbase: Use .data() instead of .c_str() on binary string (Wladimir J. van der Laan)
remove unneeded cast in rpcmisc.cpp
Merge pull request #4882

faadbe1 remove unneeded cast in rpcmisc.cpp (Philip Kaufmann)
Merge pull request #4822

629d75f Combine CCoinsViewCache's HaveCoins and const GetCoins into AccessCoins. (Pieter Wuille)
Merge pull request #4867

a95b119 qt: Remove thousands separators after decimal point (Wladimir J. van der Laan)
Merge pull request #4874

21f139b qt: fix tablet crash. closes #4854. (Cory Fields)