Adopt fine-grained row-level lock options in SELECT queries

[achimnol]

Currently we have just a with_for_update flag argument for several fetch functions for the sessions table.

Let's expand them to accept more fine-grained options like skip_locked, read, etc. to reduce the potential of serialization failures.
(ref: Teams thread)

For example, skip_locked option could be useful to make the scheduler logic reentrant with less database overheads, by fetching disjoint sets of pending sessions excluding those being handled in prior overlapping transactions and belonging to different resource groups (#323).