[data] [streaming] Enable streaming executor by default

[ericl]

Flip the feature flag for streaming executor by default, and fix any issues that occur.

[jianoaix]

Item list to resolve (updating in progress):

• Support ds.split() ([data] [streaming] Support streaming splitted datasets in AIR #32605)
• Pandas import race condition ([core] [data] Pandas has no attribute 'core' likely due to race with import thread #32435): note it's more apparent in streaming execution mode
• Autoscaling support in streaming mode ([data] [streaming] Support autoscaling with Data streaming execution #32745)
• CI passed
• Nightly passed