dict.c: introduce dictUnlink().

Notes by @antirez:

This patch was picked from a larger commit by Oran and adapted to change
the API a bit. The basic idea is to avoid double lookups when there is
to use the value of the deleted entry.

BEFORE:

    entry = dictFind( ... ); /* 1st lookup. */
    /* Do somethjing with the entry. */
    dictDelete(...);         /* 2nd lookup. */

AFTER:

    entry = dictUnlink( ... ); /* 1st lookup. */
    /* Do somethjing with the entry. */
    dictFreeUnlinkedEntry(entry); /* No lookups!. */

src/module.c

@@ -3085,7 +3085,8 @@ int moduleUnload(sds name) {
 
     /* Remove from list of modules. */
     serverLog(LL_NOTICE,"Module %s unloaded",module->name);
-    dictDeleteNoFree(modules,module->name);
+    dictDelete(modules,module->name);
+    module->name = NULL; /* The name was already freed by dictDelete(). */
     moduleFreeModuleStructure(module);
 
     return REDISMODULE_OK;