And implement 'ApproximateBestBlockDepth()' to estimate
the distance, in blocks, between the best-known block
and the network chain tip. Utilizing the best-block time
and the chainparams blocks spacing to approximate it.

This former one will be moved to the peer manager class in the
following-up commit.

No behavior change. Just an intermediate refactoring.

By relocating the peer desirable services flags into the peer
manager, we allow the connections acceptance process to handle
post-IBD potential stalling scenarios.

In the follow-up commit(s), the desirable service flags will be
dynamically adjusted to detect post-IBD stalling scenarios (such
as a +48-hour inactive node that must prefer full node connections
instead of limited peer connections because they cannot provide
historical blocks). Additionally, this encapsulation enable us
to customize the connections decision-making process based on
new user's configurations in the future.

Introduces functionality to detect when limited peers connections
are desirable or not. Ensuring that the new connections desirable
services flags stay relevant throughout the software's lifecycle.
(Unlike the previous approach, where once the validation IBD flag
was set, the desirable services flags remained constant forever).

This will let us recover from stalling scenarios where the node had
successfully synced, but subsequently dropped connections and remained
inactive for a duration longer than the limited peers threshold (the
timeframe within which limited peers can provide blocks). Then, upon
reconnection to the network, the node may end up only establishing
connections with limited peers, leading to an inability to synchronize
the chain.

This also fixes a possible limited peers threshold violation during IBD,
when the user configures `-maxtipage` further than the BIP159's limits.
This rule violation could lead to sync delays and, in the worst-case
scenario, trigger the same post-IBD stalling scenario (mentioned above)
but during IBD.