Merge 'core: wal transaction start' from Pere Diaz Bou

This pr adds support for multiple readers and a single writer with a
custom made lock called `LimboRwLock`. Basically there are 5 allowed
read locks which store the max frame allowed in that "snapshot" and any
reader will try to acquire the biggest one possible. Writer will just
try to lock the `write_lock` and if not successful, it will return busy.
The only checkpoint mode supported for now is `PASSIVE` but it should be
trivial to add more modes.
This needs testing, but I will do it in another PR. I just wanted to do
it in another PR.

Closes #544

simulator/main.rs

@@ -379,6 +379,9 @@ fn get_all_rows(
             RowResult::Done => {
                 break;
             }
+            RowResult::Busy => {
+                // for now let's retry?
+            }
         }
     }
     Ok(out)