Cache streams for queries

[simolus3]

When a query has the same sql and variables, we can assume that it will always yield the same data and should consider them to be equal. Thus, when two equal queries construct a stream, we should only create one stream for both queries.
This will also make moor work more efficient in combination with something like

StreamBuilder(
  stream: db.select(users).watch(),
  builder: //...
)

As each invocation of watch would no longer return a different stream.