Support event streams

[jclark]

Build on current support for streams and queries to support processing unbounded streams of timestamped events.

There are a number of pieces needed to make this work, including

• support for timestamp as a new basic type Timestamp datatype #280
• a way for a record to have a timestamp field that is immutable (relates to Better control over immutability #189)
• a producer/consumer model for creating streams: producer strands can add to a queue that a consumer strand will access using a stream (Producer/consumer model for creating streams #747)
• a way to distinguish a stream as being unbounded (Change 2nd type parameter of stream to support unbounded streams #453)
• windowed query
  • join (Add query clause for inner joins #435, Add query clause for outer joins #436) and grouping (Add query clauses for grouping and aggregation #441) usually need to look at the whole input to produce output
  • windowed query allows these clauses to use timestamps to produce output before seeing the whole input
  • stream probably needs some sort of watermark concept to make windowed query work

We need to create a separate proposal to deal with all of this.