share block handle between entry indexes

[tabokie]

Ref #205

stress results show 62.5% improvement compared to #206, 70% compared to v0.1.0:

# ./target/release/stress --path /data3/re_bench --time 60 --regions 1000 --write-threads 1 --purge-interval 0 --compact-count 0 --write-without-sync
Memory: 2625MiB
[write]
Throughput(QPS) = 32942.22
Latency(μs) min = 8, avg = 15.31, p50 = 12, p90 = 25, p95 = 30, p99 = 45, p99.9 = 105, max = 3681
Fairness = 100.0%
Write Bandwidth = 24.6MiB/s

[codecov[bot]]

Codecov Report

Merging #208 (a1f1f5c) into master (2fe0c36) will increase coverage by 0.01%. The diff coverage is 98.92%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   96.98%   96.99%   +0.01%     
==========================================
  Files          28       28              
  Lines        7687     7858     +171     
==========================================
+ Hits         7455     7622     +167     
- Misses        232      236       +4     

Impacted Files	Coverage Δ
src/file_pipe_log/pipe.rs	99.05% <ø> (ø)
src/memtable.rs	99.10% <98.92%> (-0.03%)	:arrow_down:
src/pipe_log.rs	94.11% <100.00%> (ø)
src/consistency.rs	91.11% <0.00%> (-2.23%)	:arrow_down:
src/codec.rs	98.34% <0.00%> (-0.34%)	:arrow_down:

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