Add a hard limit to the transaction size

[balegas]

Electric loads entire transactions from Postgres logical replication into memory before starting to write to shape logs. For very large transactions this might exhaust memory, crashing the server.

While we don't have a way to handle arbitrarily large transactions, we shall set a limit to the amount of memory a transaction can use and throw an erros.

Handling errors is not trivial, since Electric can't continue consuming the replication stream. Dropping all shapes and recreating the replication slot might be a way of getting out of that state.

Exhausting memory shouldn't happen very often and does not affect initial sync of shapes

[KyleAMathews]

Nice — yeah if we can't handle it gracefully then we really have no alternative but to dropping the transaction and killing all shapes and starting over.

We could probably set the limit based on the amount of memory of the server? If someone has trouble with big transactions then the fix is just getting a bigger box.