refactor(storage): add Send bound to NextFuture in StateStoreIter

[lmatz]

What's changed and what's your intention?

As title.

Related details can be found in the issue #1930 and PR #1956.

In essence, we need this Send bound to use iter in some async functions.

Refer to a related PR or issue link (optional)

closes #1930

[codecov]

Codecov Report

Merging #1957 (b0f442e) into main (d940c0b) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1957   +/-   ##
=======================================
  Coverage   70.91%   70.92%           
=======================================
  Files         619      619           
  Lines       79941    79941           
=======================================
+ Hits        56694    56696    +2     
+ Misses      23247    23245    -2     

Flag	Coverage Δ
rust	70.92% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/storage/src/hummock/state_store.rs	70.50% <ø> (ø)
src/storage/src/memory.rs	71.50% <ø> (ø)
src/storage/src/monitor/monitored_store.rs	2.81% <ø> (ø)
src/storage/src/panic_store.rs	0.00% <ø> (ø)
src/storage/src/rocksdb_local_mock.rs	0.00% <ø> (ø)
src/storage/src/store.rs	100.00% <ø> (ø)
src/storage/src/tikv_mock.rs	0.00% <ø> (ø)
.../src/executor/managed_state/aggregation/extreme.rs	90.26% <0.00%> (+0.26%)	⬆️

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