Merge stream.read_ahead and stream.read_ahead_limit options (backport #14894)

[mergify]

Read-ahead can be controlled by a single option, with zero acting the same as false.

This is the companion change for rabbitmq/osiris#199 - see extra context there. This replaces the config option in #14888.

# prevent any read-ahead
stream.read_ahead = false
# tune read-ahead limit
stream.read_ahead = 8kiB

---

This is an automatic backport of pull request #14894 done by [Mergify](https://mergify.com).

[the-mikedavis]

Oh, I guess my last backport I messed up the conflicts and introduced a duplicate definition

rabbitmq-server/deps/rabbit/priv/schema/rabbit.schema

Lines 2779 to 2795 in 7e43377

	{mapping, "stream.read_ahead", "rabbit.stream_read_ahead",
	[{datatype, {enum, [true, false]}}]}.
	{mapping, "stream.read_ahead_limit", "rabbit.stream_read_ahead_limit", [
	{datatype, [integer, string]},
	{validators, ["is_supported_information_unit"]}
	]}.
	{mapping, "cluster_tags.$tag", "rabbit.cluster_tags", [
	{datatype, [binary]}
	]}.
	%% See github.com/rabbitmq/osiris#192 and https://github.com/rabbitmq/rabbitmq-server/commit/9f162dfd01c1516d168e7d1fad39d33a929756e5
	%% Enables read-head
	{mapping, "stream.read_ahead", "rabbit.stream_read_ahead",
	[{datatype, {enum, [true, false]}}]}.

Working on the fix...

[the-mikedavis]

I accidentally moved or duplicated this last time

[the-mikedavis]

Same here, this was duplicated, probably because of conflicts