Fixed map_common_prefixes queries

[dannyzaken]

Signed-off-by: Danny Zaken dannyzaken@gmail.com

Explain the changes

• Changed pl/sql function map_common_prefixes to fetch all required entries in one call.
• instead of querying for 1000 keys and then extracting the common prefixes from it (can be very few common prefixes if there are a lot of objects in that "dir"), it iterates over the objectmds until reaching the max keys or the end.
• To avoid unnecessary scanning of objects - if we find a common prefix, we set the marker to be the next possible value for a common prefix (by incrementing the last character by 1)
• These changes rely on the DB having collation of LC_COLLATE = 'C'. it will not work on DBs with utf8 collation

Issues: Fixed #xxx / Gap #xxx

1. More cleanup can be done after this change. for example, it is probably unnecessary to redo the list_object queries in the object_server, since now it should return all of the required count
   

   noobaa-core/src/server/object_services/object_server.js

   Lines 1028 to 1031 in 5b6bcb5

   	while (!state.done) {
   	const results = await MDStore.instance().list_objects(state);
   	_list_add_results(state, results);
   	}

Testing Instructions:

• Doc added/updated
• Tests added

[liranmauda]

@dannyzaken Have we tested Upgrade?
In the past, we had issues when we changed sql_functions

[liranmauda]

Other then the comment, LGTM