[Feat] Add dram pool buffer manager

[Infinite666]

Purpose

1. Implement BufferManager of dram pool, it contains multiple BufferPool for different buffer sizes. It helps to choose BufferPool based on the given size.
2. Complete the TODOs relied on BufferManager in metadata management.
   TODO: Adaption for SIMU env to run gtest in pipeline.

Modifications

1. buffer.h: Common Buffer struct for BufferManager and Entry.
2. buffer_manager.cc/.h: Implement BufferManager class.
3. eviction_policy.h: Modify the return value of GetEvictionResults from BlockId to Entry for the later free of buffer slot.
4. metadata.cc/.h: Move the gc thread from MetadataManager to DramPoolServer, so that all the background threads are maintained by DramPoolServer.
5. drampool_server.cpp/.h: Replaced BufferPoolList by BufferManager.

Test

[==========] Running 71 tests from 5 test suites.
[----------] Global test environment set-up.
[----------] 19 tests from UCLruEvictionPolicyTest (5 ms total)
[----------] 22 tests from UCShardMetadataTest (0 ms total)
[----------] 4 tests from UCMetadataManagerTest (6 ms total)
[----------] 14 tests from UCPosEvictionPolicyTest (0 ms total)
[----------] 12 tests from UCTtlEvictionPolicyTest (0 ms total)

[----------] Global test environment tear-down
[==========] 71 tests from 5 test suites ran. (4660 ms total)
[  PASSED  ] 71 tests.

[nrj868]

lgtm