assign sequence numbers after initial processing

assign sequence numbers after initial processing

When re-ordering the execution of JOIN and LOCK operations (e.g., when
they attempt to execute when not enabled), the sequence numbers became
out of order. Also, RMW operations would cause sequence numbers to jump
by 2 every time.

This fix arranges initialization such that a ModelAction is only
assigned a sequence number after it passes some initial checks to ensure
that it will be executed and utilized immediately. Otherwise, it will
either be discarded or held pending until it is enabled.