snapuserd: Add I/O path support for variable block size

The flow of I/O path is as follows:

1: When there is a I/O request for a given sector, we first
check the in-memory COW operation mapping for that sector.

2: If the mapping of sector to COW operation is found, then the
existing I/O path will work seamlessly. Even if the COW operation
encodes multiple blocks, we will discard the remaining data.

3: If the mapping of sector to COW operation is not found:
    a: Find the previous COW operation as the vector has sorted sectors.

    b: If the previous COW operation is a REPLACE op:
        i: Check if the current sector is encoded in the previous COW
	operations compressed block.

	ii: If the sector falls within the range of compressed blocks,
	retrieve the block offset.

	iii: De-compress the COW operation based on the compression
	factor.

	iv: memcpy the data based on the block offset.

	v: cache the COW operation pointer as subsequent I/O requests
	are sequential and can just be a memcpy at the correct offset.
    c: If the previous COW operation is not a REPLACE op or if the
       requested sector does not fall within the compression factor
       of the previous COW operation, then fallback and read the data
       from base device.

Snapshot-merge:

During merge of REPLACE ops, read the entire op in one shot, de-compress
multiple blocks and write all the blocks in one shot.

Performance:

go/variable-block-vabc-perf covers detail performance runs
on Pixel 6 for full and incremental OTA.

Bug: 319309466

Test: snapuserd_test covers all the I/O path with various block sizes.
About 252 cases with all combinations and tunables.

[==========] 252 tests from 4 test suites ran. (702565 ms total)
[  PASSED  ] 252 tests.

On Pixel 6:

=======================================
COW Writer V3:

for i in full, incremental OTA
   for j in 4k, 16k, 32k, 64k, 128, 256k
      for k in lz4, zstd, gz
	 install OTA, reboot, verify merge
=======================================
COW Writer V2:

for i in full, incremental OTA
  for j in 4k
    for k in lz4, zstd, gz
      install OTA, reboot, verity merge

=====================================

Change-Id: I4c3b5c3efa0d09677568b4396cc53db0e74e7c99
Signed-off-by: Akilesh Kailash <akailash@google.com>