When the toppartitions operation gathers results, it copies partition
keys with their schema_ptr:s. When these schema_ptr:s are copies
or destroyed, they can cause leaks or premature frees of the schema
in its original shard since reference count operations in are not atomic.
Fix that by converting the schema_ptr to a global_schema_ptr during
transportation.
Fixes#5104 (direct bug)
Fixes#5018 (schema prematurely freed, toppartitions previously executed on that node)
Fixes#4973 (corrupted memory pool of the same size class as schema, toppartitions previously executed on that node)
Tests: new test added that fails with the existing code in debug mode,
manual toppartitions test
5b0e48f25b