Merge write coalescing configuration options

[muzarski]

#1297 introduced WriteCoalescingDelay enum along with write_coalescing_delay session configuration option. It's heavily dependent on enable_write_coalescing configuration flag.

These two config options should probably be merged into one (represented as Option<WriteCoalescingDelay> or something similar).

This is a small API cleanup we can do as a part of 2.0 (can't do it before, because of API breaking changes).