various optimizations (#704)

- Improvement: Validate chunk hashes when dechunking files in oplog import
- Improvement: Use stream decompression when dechunking files
- Improvement: When assembling blocks for oplog export, make sure we keep under/at block size limit
- Improvement: Make cancelling of oplog import more responsive
- Improvement: Use decompress to composite to avoid allocating a new memory buffer for uncompressed chunks during oplog import
- Improvement: Reduce memory buffer size and allocate it on demand when writing multiple chunks to block store
- Improvement: Reduce lock contention when fetching/checking existence of chunks in block store

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