Use stree for message block fss (#5559)

Should provide some deduplication of long subjects in memory when
message blocks are loaded in.

Signed-off-by: Neil Twigg <[email protected]>

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Signed-off-by: Neil Twigg <[email protected]>
Signed-off-by: Derek Collison <[email protected]>
Co-authored-by: Derek Collison <[email protected]>

Author: neilalexander