[enhancement](load) add slow log for memtable flush

[zhannngchen]

Proposed changes

Issue Number: close #xxx

Problem summary

Some slow load is caused by close wait for too long time, we can print log if close wait too slow.

Checklist(Required)

• Does it affect the original behavior
• Has unit tests been added
• Has document been added or modified
• Does it need to update dependencies
• Is this PR support rollback (If NO, please explain WHY)

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[zhannngchen]

run buildall

[hello-stephen]

TeamCity pipeline, clickbench performance test result:
the sum of best hot time: 34.21 seconds
stream load tsv: 451 seconds loaded 74807831229 Bytes, about 158 MB/s
stream load json: 25 seconds loaded 2358488459 Bytes, about 89 MB/s
stream load orc: 73 seconds loaded 1101869774 Bytes, about 14 MB/s
stream load parquet: 31 seconds loaded 861443392 Bytes, about 26 MB/s
https://doris-community-test-1308700295.cos.ap-hongkong.myqcloud.com/tmp/20230322024416_clickbench_pr_118319.html

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[zhannngchen]

run buildall

[dataroaring]

LGTM