Make comparison operator member functions const

[stbergmann]

...which avoids overload resolution ambiguities in C++20, when a synthesized
candidate of operator == for a reversed-argument rewrite conflicts with the
actual operator ==, due to the asymmetric const-ness of the implicit object
parameter and the RHS parameter. (As observed with recent Clang 10 trunk with
-std=c++2a when building firebird as part of LibreOffice:

workdir/UnpackedTarball/firebird/src/jrd/inf.cpp:1139:62: error: use of overloaded operator '!=' is ambiguous (with operand types 'RuntimeStatistics::Iterator' and 'Jrd::RuntimeStatistics::Iterator')
for (RuntimeStatistics::Iterator iter = stats.begin(); iter != stats.end(); ++iter)
~~~~ ^ ~~~~~~~~~~~
workdir/UnpackedTarball/firebird/src/jrd/../dsql/../jrd/RuntimeStatistics.h:283:8: note: candidate function
bool operator!=(const Iterator& other)
^
workdir/UnpackedTarball/firebird/src/jrd/../dsql/../jrd/RuntimeStatistics.h:278:8: note: candidate function
bool operator==(const Iterator& other)
^
workdir/UnpackedTarball/firebird/src/jrd/../dsql/../jrd/RuntimeStatistics.h:278:8: note: candidate function (with reversed parameter order)

)