snapuserd: Use io_uring api's for snapshot merge

using io_uring READ/WRITE opcodes for snapshot merge.
Specifically, it is used only for readahead and ordered ops
code path.

Snapshot merge perf:

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Incremental OTA of 300M between two git_master branches on Pixel 6:

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On Android S (with dm-snapshot): ~15 minutes:

update_engine: [INFO:cleanup_previous_update_action.cc(330)] Merge finished with state MergeCompleted.
update_engine: [INFO:cleanup_previous_update_action.cc(130)] Stopping/suspending/completing CleanupPreviousUpdateAction
update_engine: [INFO:cleanup_previous_update_action.cc(501)] Reporting merge stats: MergeCompleted in 926508ms (resumed 0 times), using 0 bytes of COW image.

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On Android T (with io_uring: ~38 seconds):

update_engine: [INFO:cleanup_previous_update_action.cc(330)] Merge finished with state MergeCompleted.
update_engine: [INFO:cleanup_previous_update_action.cc(130)] Stopping/suspending/completing CleanupPreviousUpdateAction
update_engine: [INFO:cleanup_previous_update_action.cc(501)] Reporting merge stats: MergeCompleted in 38868ms (resumed 0 times), using 0 bytes of COW image.

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Bug: 202784286
Test: Full/Incremental OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I24ed3ae16730d0a18be0350c162dc67e1a7b74e1