leveled recalc strategy should default to enabled

[martinsumner]

In riak 2.9.2 the option to use a recalc strategy for rebuilding the key store in leveled was introduced:

riak_kv/priv/riak_kv.schema

Lines 1445 to 1463 in feced03

	%% @doc Enable the `recalc` compaction strategy within the leveled backend in
	%% riak. The default (when disabled) is `retain`, but this will leave
	%% uncollected garbage within the, journal.
	%% It is now recommended from Riak KV 2.9.2 to consider the `recalc` strategy.
	%% This strategy has a side effect of slower startups, and slower recovery
	%% from a wiped ledger - but it will not keep an overhead of garbage within
	%% the Journal.
	%% It should be possible to move from `retain` to `recalc` via configuration
	%% change. However, it is not possible to switch from `recalc` back to
	%% `retain`. This switch can only be made for new nodes receiving data
	%% through riak transfers (not inheriting data on disk).
	%% The default `retain` strategy retains a history of key changes in the
	%% journal, whereas the `recalc` strategy discards that history, but will redo
	%% a diff_index_specs calculation when reloading each object.
	{mapping, "leveled_reload_recalc", "riak_kv.leveled_reload_recalc", [
	{datatype, {flag, enabled, disabled}},
	{default, disabled},
	{commented, enabled}
	]}.

The release notes guided that this option should be enabled, though it was disabled by default. It was disabled by default as there was no way of reverting back from recalc to retain, and so the intention was to give users time to test the change before enabling the configuration.

Sufficient time has now passed, and the configuration has been tested over many years in production environments - it should now be enabled by default.