snapuserd: Cut down worker threads

We don't need 4 threads per partition all the time.
Cut down to 1 thread for all purposes except during boot time.

During boot, we need multiple worker threads to serve
I/O's to speed up the booting process. For all other
purposes, single thread is sufficient. It will cut
down memory usage ~25MB.

Bug: 214340811
Test: OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ib36bf44c63676d3f3169fbda04b7fc3d1760cfbf