[mlir][StorageUniquer] Use allocators per thread instead of per shard

This greatly reduces the number of allocators we create, while still
retaining thread safety. Reducing the number of allocators is much
better for locality and memory usage; this revision drops memory
usage for some MLIR heavy workloads (with lots of attributes/types)
by >=5%. This is due to the observation that the number of threads
is effectively always smaller than the number of parametric attributes/types.

Differential Revision: https://reviews.llvm.org/D145991