ScriptSession API to ensure snapshots/diffs of bindings are always consistent.

[niloc132]

Original title/topic: Generalize script session "deltas", probably by letting the script session entirely control snapshots internally, rather than expose this directly, such as by returning changes when running script code. This will also deal with some potential races, making it the script session implementation's responsibility to not lose track of changes.

Split off from #2453 to be its own issue.

This was found to be broken, where assigning/nulling a binding rapidly can break the application service's internal state, by missing changes and emitting inconsistent updates:

from deephaven_server import Server
s = Server()
s.start()

import time
from deephaven import empty_table, time_table

while True:
    t = time_table("PT1m")
    time.sleep(.04)
    t = None
    time.sleep(.04)

Usually takes 10s-1minute to fail.