[Feature] Improve scan performance in hot read paths

Start by profiling the manifest reader's StructArray::fields() calls and the Avro decoder's ArrayBuilder::type() calls in concurrent scan paths. Identify the appropriate batch, reader, or builder lifetime for immutable Arrow metadata, then verify caches are invalidated when the corresponding Arrow object tree is replaced. Done means reduced shared-pointer synchronization and reference-counting overhead under concurrent scans.

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Motivation

Recent profiling of highly concurrent scans has revealed several performance bottlenecks caused by
repeated operations on Arrow-returned shared_ptr objects in hot read loops.

Two significant cases have been identified:

1. Manifest readers repeatedly call StructArray::fields() while processing individual rows. This
   copies shared_ptr<Array> objects and, with GCC 8.3's libstdc++, can introduce substantial lock
   contention through _Sp_locker, pthread_mutex_lock, and futex waits when multiple workers read
   manifests concurrently.

2. Avro decoding calls ArrayBuilder::type() for every integer and timestamp value. Because this
   method returns std::shared_ptr<DataType> by value, concurrent scans repeatedly modify reference
   counts on shared Arrow primitive data types, causing cache-line contention. Profiling showed
   ArrayBuilder::type() and shared-pointer release operations accounting for a large proportion of
   samples after the manifest bottleneck was removed.

This issue tracks the broader effort to identify and eliminate similar shared-pointer operations
from scan hot paths. The goal is to cache immutable Arrow metadata at an appropriate batch, reader,
or builder lifetime, while ensuring caches are invalidated whenever the corresponding Arrow object
tree is replaced.

The expected outcome is lower synchronization and reference-counting overhead under concurrent
scans, allowing CPU time to return to actual decoding, memory copying, and buffer management.

Solution

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Anything else?

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Are you willing to submit a PR?

• I'm willing to submit a PR!