(core) add GVISOR_LIMIT_MEMORY to cap memory available in sandbox

Summary:
This allows limiting the memory available to documents in the sandbox when gvisor is used. If memory limit is exceeded, we offer to open doc in recovery mode. Recovery mode is tweaked to open docs with tables in "ondemand" mode, which will generally take less memory and allow for deleting rows.

The limit is on the size of the virtual address space available to the sandbox (`RLIMIT_AS`), which in practice appears to function as one would want, and is the only practical option. There is a documented `RLIMIT_RSS` limit to `specifies the limit (in bytes) of the process's resident set (the number of virtual pages resident in RAM)` but this is no longer enforced by the kernel (neither the host nor gvisor).

When the sandbox runs out of memory, there are many ways it can fail. This diff catches all the ones I saw, but there could be more.

Test Plan: added tests

Reviewers: alexmojaki

Reviewed By: alexmojaki

Subscribers: alexmojaki

Differential Revision: https://phab.getgrist.com/D3398

sandbox/grist/engine.py

@@ -866,6 +866,9 @@ class Engine(object):
         raise self._cell_required_error  # pylint: disable=raising-bad-type
       self.formula_tracer(col, record)
       return result
+    except MemoryError:
+      # Don't try to wrap memory errors.
+      raise
     except:  # pylint: disable=bare-except
       # Since col.method runs untrusted user code, we use a bare except to catch all
       # exceptions (even those not derived from BaseException).