Fix bugs for RTC2RTMP.

[xiaozhihong]

1. Cache IDR frame's rtp timestamp instead of avsync timestamp.
2. Cache clock rate calculate by sender report.
3. Using srs_rtp_seq_distance instead of direct minus.
4. Add utest of av timestamp sync when duplicated sender report.

[chen-guanghua]

1. key_frame_ts_: The meaning expressed is the timestamp of the RTP key frame. The variable is defined as rtp_key_frame_ts_. No need to add comments, as the meaning is clear from the variable name.
2. If additional comments are required, they can be added as a new line above the variable. Follow the same formatting as other comments. 😄

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[chen-guanghua]

fpclassify was introduced in C++11. Should we consider older versions of gcc?

https://en.cppreference.com/w/cpp/numeric/math/fpclassify

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[xiaozhihong]

This is a function in the standard C library, and there is also a corresponding version in std:: library.

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[codecov-commenter]

Codecov Report

Merging #2768 (ea569ab) into 4.0release (bc06394) will increase coverage by 0.03%.
The diff coverage is 74.68%.

@@              Coverage Diff               @@
##           4.0release    #2768      +/-   ##
==============================================
+ Coverage       60.11%   60.15%   +0.03%     
==============================================
  Files             121      121              
  Lines           50896    50947      +51     
==============================================
+ Hits            30597    30648      +51     
  Misses          20299    20299              

| Impacted Files | Coverage Δ | |'

Translated to English while maintaining the markdown structure:

'| Impacted Files | Coverage Δ | |
|---|---|---|
| trunk/src/app/srs_app_rtc_source.hpp | 9.09% <ø> (ø) | |'

Translated to English while maintaining the markdown structure:

| trunk/src/app/srs_app_rtc_source.hpp | 9.09% <ø> (ø) | |
| trunk/src/app/srs_app_rtc_source.cpp | 12.47% <37.50%> (+0.24%) | ⬆️ |
| trunk/src/utest/srs_utest_rtc.cpp | 99.39% <100.00%> (+0.04%) | ⬆️ |

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| Δ = absolute <relative> (impact), ø = not affected, ? = missing data |

Translated to English while maintaining the markdown structure:

| Δ = absolute <relative> (impact), ø = not affected, ? = missing data |

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