Prefer 'unsigned int' for loop index variables tested against ::size()

Prefer 'unsigned int' for loop index variables tested against ::size()

C++ STL ::size() generally returns unsigned, which implies that "int idx"
style of loop variable will generate a signed-vs-unsigned comparison warning
when testing the loop exit condition "idx < blah.size()"

Update areas of the bitcoin code where loop variables may be more properly and
correctly defined as unsigned.