Removes the Boost case_conv.hpp dependency.

This commit removes the `boost/algorithm/string/case_conv.hpp` dependency from the project. It replaces the `boost::to_lower` and `boost::to_upper` functions with custom functions that are locale independent and ASCII deterministic.

test/lint/lint-includes.sh

@@ -47,7 +47,6 @@ fi
 
 EXPECTED_BOOST_INCLUDES=(
     boost/algorithm/string.hpp
-    boost/algorithm/string/case_conv.hpp
     boost/algorithm/string/classification.hpp
     boost/algorithm/string/replace.hpp
     boost/algorithm/string/split.hpp

test/lint/lint-locale-dependence.sh

@@ -11,11 +11,9 @@ KNOWN_VIOLATIONS=(
     "src/dbwrapper.cpp:.*vsnprintf"
     "src/httprpc.cpp.*trim"
     "src/init.cpp:.*atoi"
-    "src/netbase.cpp.*to_lower"
     "src/qt/rpcconsole.cpp:.*atoi"
     "src/qt/rpcconsole.cpp:.*isdigit"
     "src/rest.cpp:.*strtol"
-    "src/rpc/server.cpp.*to_upper"
     "src/test/dbwrapper_tests.cpp:.*snprintf"
     "src/test/getarg_tests.cpp.*split"
     "src/torcontrol.cpp:.*atoi"