feature(advanced RPC compression): enable dictionary training and ZSTD compression

[michoecho]

This patch:

• enables dictionary training (by changing rpc_dict_training_when from the default value of 'never' to 'when_leader),
• increases (by 4x) the frequency of dictionary training and updates, to stress the feature more,
• enables ZSTD internode compression (provided that internode_compression is also enabled) by giving it a nonzero CPU usage limit.

However, this patch doesn't enable internode_compression itself, because it's a performance-affecting option. To actually put the feature to use, a test must enable internode_compression on its own.

Testing

• Syntax check.
• Untested.

PR pre-checks (self review)

• [x] I added the relevant backport labels
• [x] I didn't leave commented-out/debugging code

Refs #7364