Refactoring: improve luaCreateFunction() API.

The function in its initial form, and after the fixes for the PSYNC2
bugs, required code duplication in multiple spots. This commit modifies
it in order to always compute the script name independently, and to
return the SDS of the SHA of the body: this way it can be used in all
the places, including for SCRIPT LOAD, without duplicating the code to
create the Lua function name. Note that this requires to re-compute the
body SHA1 in the case of EVAL seeing a script for the first time, but
this should not change scripting performance in any way because new
scripts definition is a rare event happening the first time a script is
seen, and the SHA1 computation is anyway not a very slow process against
the typical Redis script and compared to the actua Lua byte compiling of
the body.

Note that the function used to assert() if a duplicated script was
loaded, however actually now two times over three, we want the function
to handle duplicated scripts just fine: this happens in SCRIPT LOAD and
in RDB AUX "lua" loading. Moreover the assert was not defending against
some obvious failure mode, so now the function always tests against
already defined functions at start.

src/rdb.c

@@ -1679,7 +1679,7 @@ int rdbLoadRio(rio *rdb, rdbSaveInfo *rsi) {
                 if (rsi) rsi->repl_offset = strtoll(auxval->ptr,NULL,10);
             } else if (!strcasecmp(auxkey->ptr,"lua")) {
                 /* Load the script back in memory. */
-                if (luaCreateFunction(NULL,server.lua,NULL,auxval,1) == C_ERR) {
+                if (luaCreateFunction(NULL,server.lua,auxval) == NULL) {
                     rdbExitReportCorruptRDB(
                         "Can't load Lua script from RDB file! "
                         "BODY: %s", auxval->ptr);