CAS-14759: Fix potential lock contention in SIImageStore::makeImageBeamSet
This commit addresses a performance bottleneck where `tclean` would stall
for extended periods (often hours) during the initialization and
re-fitting phases of large spectral cubes with many MPI ranks.
Within `makeImageBeamSet()`, a `LatticeLocker` read-lock on the PSF image
was instantiated and subsequently destroyed on every single combined
channel and polarization iteration. For a 60,000 channel cube, this resulted
in 60,000 individual, sequential Lustre metadata transactions (lock requests
and releases) per process.
The `LatticeLocker` is now hoisted outside the nested loops, allowing the
process to obtain a single read-lock that persists for the duration of the
fitting iterations. This condenses millions of filesystem lock updates
across an MPI cluster down to exactly one, significantly reducing I/O
synchronization overhead during `mpicasa` execution.
