[opt](stats) Use single connect context for each olap analyze task

[Kikyou1997]

1. Add some comment for codes of AnalysisManager
2. Fix potential NPE caused by deleting a running analyze job
3. Use single connect context for each olap analyze task

Proposed changes

Issue Number: close #xxx

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...

[Kikyou1997]

run buildall

[hello-stephen]

(From new machine)TeamCity pipeline, clickbench performance test result:
the sum of best hot time: 44.63 seconds
stream load tsv: 509 seconds loaded 74807831229 Bytes, about 140 MB/s
stream load json: 20 seconds loaded 2358488459 Bytes, about 112 MB/s
stream load orc: 65 seconds loaded 1101869774 Bytes, about 16 MB/s
stream load parquet: 31 seconds loaded 861443392 Bytes, about 26 MB/s
insert into select: 29.4 seconds inserted 10000000 Rows, about 340K ops/s
storage size: 17162153493 Bytes

[Kikyou1997]

run buildall

[hello-stephen]

(From new machine)TeamCity pipeline, clickbench performance test result:
the sum of best hot time: 44.94 seconds
stream load tsv: 511 seconds loaded 74807831229 Bytes, about 139 MB/s
stream load json: 20 seconds loaded 2358488459 Bytes, about 112 MB/s
stream load orc: 65 seconds loaded 1101869774 Bytes, about 16 MB/s
stream load parquet: 30 seconds loaded 861443392 Bytes, about 27 MB/s
insert into select: 29.3 seconds inserted 10000000 Rows, about 341K ops/s
storage size: 17162095343 Bytes

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.