gh-149584: Fix excessive overhead in the Tachyon profiler regarding the cache behavior

[pablogsal]

Some ideas after the discussion in the issue with @maurycy. The profiler was spending too much time on repeated remote-memory bookkeeping, full remote page reads for small fixed-size structs, repeated remote writes of unchanged frame-cache state, and Python object allocation churn on steady-state frame-cache hits.

This PR improves the profiler by:

• reading hot interpreter/thread/frame structs with exact remote reads instead of page-cache reads
• tracking the number of live page-cache entries so cache clear/search only scans the used prefix
• batching predicted interpreter/thread/frame reads with process_vm_readv() on Linux
• reusing cached FrameInfo and thread id objects when frame-cache hits dominate

The last_profiled_frame remote-write suppression is already present in current upstream/main, so this branch keeps that baseline behavior and builds on top of it.

Benchmark

Benchmarked with:

./python Tools/inspection/benchmark_external_inspection.py --duration 3

For the per-commit measurements, I used the same benchmark workload in quiet mode with cache_frames=True and all_threads=True.

Step	Work rate	Sample rate	Avg call time	Incremental win
upstream/main baseline	382,593/s	363,963/s	2.614 us	-
Exact reads for hot structs	549,741/s	514,642/s	1.819 us	+167,148/s (+43.7%)
Track live page-cache entries	747,946/s	688,282/s	1.337 us	+198,205/s (+36.1%)
Batch predicted remote reads	897,703/s	806,084/s	1.114 us	+149,756/s (+20.0%)
Reuse profiler result objects	1,187,314/s	1,047,698/s	0.842 us	+289,611/s (+32.3%)

Final benchmark using the benchmark script:

Metric	Baseline	Final	Change
Work rate	382,593/s	1,180,434/s	3.08x faster
Sample rate	363,963/s	1,038,965/s	2.85x faster
Avg call time	2.614 us	0.85 us	-67.5%
Success rate	99.67%	99.84%	+0.17 pp

Final benchmark output:

Average call time:   0.85 us
Work rate:           1,180,434.2 calls/sec
Sample rate:         1,038,964.9 samples/sec
Success rate:        99.84%

• Issue: _remote_debugging: reading whole pages over and over #149584

[pablogsal]

@maurycy do you mind reviewing this PR?