Send deterministic ordering into unit of work topological

Send deterministic ordering into unit of work topological

Improved the unit of work topological sorting system such that the
toplogical sort is now deterministic based on the sorting of the input set,
which itself is now sorted at the level of mappers, so that the same inputs
of affected mappers should produce the same output every time, among
mappers / tables that don't have any dependency on each other. This further
reduces the chance of deadlocks as can be observed in a flush that UPDATEs
among multiple, unrelated tables such that row locks are generated.

topological.sort() has been made "deterministic" in all cases by
using a separate list + set.

Fixes: #5735
Change-Id: I073103df414dba549e46605b394f8ccae6e80d0e