Rethink Watch

[aravindet]

TL:DR; Replace watch() with incremental read() polling

Why

The current implementation of watch() is complex to implement in providers and doesn't support back-pressure or resumption.

Steps

• Implement the new where query version semantics ("if-changed-after")
  • Specify that version is a non-negative number, and that version 0 has a special meaning
  • Implement the querying of linked paths in graffy-link and drop graffy-fill completely
  • Restrict the use of finalize() to queries with version 0 in core, pg and link
• Implement query version filtering in slice(): exclude unchanged from both known and unknown
• Implement query version filtering in pg by adding a condition on verCol
• Implement the async iterator form of read() to perform incremental polling