[2376] Rename log IDs

There are no contexts any more. Rename the log message IDs not to
contain them.

src/lib/datasrc/datasrc_messages.mes

@@ -305,14 +305,14 @@ Therefore, the zone will not be available for this process. If this is
 a problem, you should move the zone to some database backend (sqlite3, for
 example) and use it from there.
 
-% DATASRC_LOAD_CONTEXT_ERROR %1:%2: Zone '%3/%4' contains error: %5
+% DATASRC_MASTER_LOAD_ERROR %1:%2: Zone '%3/%4' contains error: %5
 There's an error in the given master file. The zone won't be loaded for
 this reason. Parsing might follow, so you might get further errors and
 warnings to fix everything at once. But in case the error is serious enough,
 the parser might just give up or get confused and generate false errors
 afterwards.
 
-% DATASRC_LOAD_CONTEXT_WARN %1:%2: Zone '%3/%4' has a potential problem: %5
+% DATASRC_MASTER_LOAD_WARN %1:%2: Zone '%3/%4' has a potential problem: %5
 There's something suspicious in the master file. This is a warning only.
 It may be a problem or it may be harmless, but it should be checked.
 This problem does not stop the zone from being loaded.

src/lib/datasrc/loader_callbacks.cc

@@ -33,7 +33,7 @@ logError(const isc::dns::Name& name, const isc::dns::RRClass& rrclass,
          bool* ok, const std::string& source, size_t line,
          const std::string& reason)
 {
-    LOG_ERROR(logger, DATASRC_LOAD_CONTEXT_ERROR).arg(source).arg(line).
+    LOG_ERROR(logger, DATASRC_MASTER_LOAD_ERROR).arg(source).arg(line).
         arg(name).arg(rrclass).arg(reason);
     if (ok != NULL) {
         *ok = false;
@@ -44,7 +44,7 @@ void
 logWarning(const isc::dns::Name& name, const isc::dns::RRClass& rrclass,
          const std::string& source, size_t line, const std::string& reason)
 {
-    LOG_WARN(logger, DATASRC_LOAD_CONTEXT_WARN).arg(source).arg(line).
+    LOG_WARN(logger, DATASRC_MASTER_LOAD_WARN).arg(source).arg(line).
         arg(name).arg(rrclass).arg(reason);
 }