Summary:
Changing `ysql_enable_write_pipelining` to true by default, and changing it from a preview flag

Also fixing 2 broken tests from enabling this flag:

`PgSingleServerRestartTest` - this test blocks replication progress with TEST_pause_update_majority_replicated and expects INSERTs to hang/fail immediately, since without pipelining a write can't be acked while majority-replication is paused. Disabling write pipelining for this test.

`PgDelayedSplitAtFollower.TestDelayedSplitAtFollower` - with write pipelining, the bulk INSERT is acked before its intents finish applying, so the flush/split can run against a tablet whose intents aren't in place yet (this results in a `No SST file at level 0` error when splitting). Adding in a WaitForTableIntentsApplied between the INSERT and the flush + split.

Test Plan: Jenkins

Reviewers: neera.mital, hsunder

Reviewed By: hsunder

Subscribers: yql, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D55777