c/partition_balancer: don't copy topic metadata on each partition visit

[ztlpn]

Previously we were copying the whole metadata (including partition assignments) just to query the topic revision.

partition_balancer_bench results before:

test                                                   iterations      median         mad         min         max      allocs       tasks        inst
partition_balancer_planner_fixture.unavailable_nodes           27    24.481ms   161.139us    24.320ms    35.441ms 1235710.104      93.852 425407989.4
partition_balancer_planner_fixture.counts_rebalancing           1      1.857s     7.421ms      1.846s      1.888s 118912546.400    7424.600 33427470288.6

and after:

test                                                   iterations      median         mad         min         max      allocs       tasks        inst
partition_balancer_planner_fixture.unavailable_nodes           91     8.695ms     1.429ms     5.078ms    10.124ms   68660.031      11.301 100617054.8
partition_balancer_planner_fixture.counts_rebalancing          19    54.006ms    56.923us    53.942ms    55.873ms 2172589.168     208.305 972473623.6

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.3.x
• v23.2.x

Release Notes

• none

[dotnwat]

1s vs 55ms is not bad!

[bharathv]

yikes, thanks for fixing it, I believe I added that snippet of code.

[vbotbuildovich]

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/46831#018e7d17-7506-404e-8304-13e5df7dc7c9

[vbotbuildovich]

/backport v23.3.x