Streaming Ingest API writing unbucketed tables

Currently the API expects the target table to be bucketed.
It creates 1 writer per bucket per connection/partition.
The simplest is to allow the API to create a single writer for unbucketed tables.
If this doesn't provide enough write throughput, the client can create another connection.

Could add a parameter to the API to specify writer parallelism for unbucketed tables. If it's set to 2 for example, the writer will write delta_x_y_0000 and delta_x_y_00001 using statementId. Maybe as a followup.