Modify default session timeout

[Aiee]

What type of PR is this?

• bug
• feature
• enhancement

What does this PR do?

1. Modify default value for session_idle_timeout_secs and client_idle_timeout_secs from 0 (never timeout) to 28800(8 hours, which is also the default timeout MySQL uses).
2. Add gflag validator for session_idle_timeout_secs which checks the value when program starts and updating value. The valid range for session_idle_timeout_secs is [1, 604800]

Which issue(s)/PR(s) this PR relates to?

Close #3305

Special notes for your reviewer, ex. impact of this fix, etc:

Additional context:

Incompatible: Users should modify their configuration, i.e. change 0 to a valid value after this PR is merged.

Ref: https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_wait_timeout

Checklist：

• Documentation affected （Please add the label if documentation needs to be modified.)
• Incompatible （If it is incompatible, please describe it and add corresponding label.）
• Need to cherry-pick （If need to cherry-pick to some branches, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes：

Please confirm whether to reflect in release notes and how to describe:

                                                            `

[codecov-commenter]

Codecov Report

Merging #3357 (0dec760) into master (fe68b0e) will decrease coverage by 0.02%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master    #3357      +/-   ##
==========================================
- Coverage   85.27%   85.24%   -0.03%     
==========================================
  Files        1276     1277       +1     
  Lines      118999   119026      +27     
==========================================
- Hits       101475   101468       -7     
- Misses      17524    17558      +34     

Impacted Files	Coverage Δ
src/graph/session/GraphSessionManager.cpp	80.79% <33.33%> (-0.82%)	⬇️
src/common/expression/ContainerExpression.cpp	90.00% <66.66%> (-2.13%)	⬇️
src/graph/executor/admin/ConfigExecutor.cpp	93.84% <90.90%> (-0.70%)	⬇️
src/clients/meta/MetaClient.cpp	76.03% <100.00%> (ø)
src/graph/service/GraphFlags.cpp	100.00% <100.00%> (ø)
src/graph/validator/test/YieldValidatorTest.cpp	91.78% <100.00%> (ø)
src/parser/test/ParserTest.cpp	100.00% <100.00%> (ø)
src/kvstore/wal/WalFileIterator.cpp	71.00% <0.00%> (-5.00%)	⬇️
src/common/base/Status.h	88.63% <0.00%> (-2.28%)	⬇️
src/kvstore/raftex/Host.cpp	71.90% <0.00%> (-2.01%)	⬇️
... and 22 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d29a5a4...0dec760. Read the comment docs.

[yixinglu]

Maybe you could cleanup some logics about zero timeout handling.

[HarrisChu]

after this pr, users should modify their configuration, i.e. change 0 to a valid value.
so I add incompatible label.

[yixinglu]

LGTM

[darionyaphet]

LGTM