Support Prepare() in BlockBasedTableIterator For MultiScan (facebook#13778)

Summary:
initial support for Prepare() to optimize the performance of MultiScan when using block-based tables. In Prepare(), we do the following:
1. Load all data blocks that will be read in multiscan to block cache
2. Pin the data blocks during the scan
3. if I/O is needed, coalesce I/Os when they are adjacent.

Pull Request resolved: facebook#13778

Test Plan:
Added a new unit test.

Benchmark:
1. Set up the DB, I use FIFO here so that files will be in L0 and iterator will use BlockBasedTableIterator directly instead of LevelIterator, where Prepare() call is not implemented yet.
```
./db_bench --benchmarks="fillseq,compact" --disable_wal=1 --threads=1 --num_levels=1 --compaction_style=2 --fifo_compaction_max_table_files_size_mb=1000 --write_buffer_size=268435456
```

2. Multi-scan: based on facebook#13765
```
./db_bench --db="/tmp/rocksdbtest-543376/dbbench" --use_existing_db=1 --benchmarks=multiscan --disable_auto_compactions=1 --seek_nexts=100 --threads=32 --duration=10 --statistics=1

multiscan_stride = 100
multiscan_size = 10
seek_nexts = 100

Main:
multiscan    :     449.386 micros/op 70562 ops/sec 10.359 seconds 730968 operations; (multscans:22999)
multiscan    :     453.606 micros/op 69433 ops/sec 10.369 seconds 719968 operations; (multscans:22999)
rocksdb.non.last.level.read.bytes COUNT : 47763519421
rocksdb.non.last.level.read.count COUNT : 21573878
Branch:
multiscan    :     332.670 micros/op 94698 ops/sec 10.285 seconds 973968 operations; (multscans:29999)
rocksdb.non.last.level.read.bytes COUNT : 111791308336
rocksdb.non.last.level.read.count COUNT : 1062942

With direct-IO:
./db_bench --db="/tmp/rocksdbtest-543376/dbbench" --use_existing_db=1 --benchmarks=multiscan --disable_auto_compactions=1 --seek_nexts=100 --threads=32 --duration=10 --statistics=1 --use_direct_reads=1

Main:
multiscan    :     586.045 micros/op 53825 ops/sec 10.366 seconds 557968 operations; (multscans:14999)
rocksdb.non.last.level.read.bytes COUNT : 69107458693
rocksdb.non.last.level.read.count COUNT : 6724651
Branch:
multiscan    :     386.679 micros/op 81282 ops/sec 10.359 seconds 841968 operations; (multscans:25999)
rocksdb.non.last.level.read.bytes COUNT : 96605800558
rocksdb.non.last.level.read.count COUNT : 918973
```

Throughput is 36% higher with non-direct IO and 50% higher with direct IO. The improvement is likely from doing less number of I/Os due to I/O coalescing during Prepare(), as shown in `rocksdb.non.last.level.read.count`. The total bytes read is more with this PR for the same reason.

3. Regular iterator:
```
./db_bench --use_existing_db=1 --db="/tmp/rocksdbtest-543376/dbbench" --benchmarks=seekrandom --disable_auto_compactions=1 --seek_nexts=10 --threads=32 --duration=10

Main:
seekrandom   :      13.014 micros/op 2456735 ops/sec 10.014 seconds 24602968 operations; 2717.8 MB/s (773999 of 773999 found)
Branch:
seekrandom   :      13.048 micros/op 2450554 ops/sec 10.013 seconds 24537968 operations; 2710.9 MB/s (772999 of 772999 found)
```
The result fluctuates but without noticeable regression.

Reviewed By: anand1976

Differential Revision: D78440807

Pulled By: cbi42

fbshipit-source-id: 80ac6fd222696fa65ac0b4b5441748be5ee0b979

Author: cbi42