[Perf][C++] Load a device's timeseries indexes with one pread instead of per-measurement reads #1074

Problem

TsFileIOReader::get_timeseries_indexes handles each requested measurement
individually: binary search in the device index node, a separate
read_file_->read (PREAD backend, so one syscall each), deserialization into a
temporary arena, then a clone into the caller's arena. For a 4500-column table
that is ~4500 preads plus ~4500 deserialize+clone.

Profiling a wide-table read showed do_load_timeseries_index at ~7.6%.

Proposed changes

Collect every measurement leaf once (get_all_leaf), read the whole contiguous
timeseries-index range with a single pread, and deserialize each index directly
into the caller's arena; build a name -> index map and return the requested
ones.

Implemented in #1072; index loading dropped from ~7.8% to ~0.2% of the query.

Related: #1072