Fix FlagsField & dissection improvements

[gpotter2]

This is a part of #1999 that can be fast tracked.
This PR:

• Small performances boost: consistent 5-10% on dissection (caching improvements & guess_payload_class)

>>> a = raw(Ether()/IP()/UDP())
>>> %timeit Ether(a)

• Fixes FlagsField very weird behavior:

before (?!?!)

>>> EIR_Hdr() / EIR_Flags(flags=["general_disc_mode", "br_edr_not_supported"])
<EIR_Hdr  type=flags |<EIR_Flags  flags=['general_disc_mode', 'br_edr_not_supported'] |>>
+ when raw(), flags was wrongly built

after

>>> EIR_Hdr() / EIR_Flags(flags=["general_disc_mode", "br_edr_not_supported"])
<EIR_Hdr  type=flags |<EIR_Flags  flags=general_disc_mode+br_edr_not_supported |>>

• Improve dissection speed by reducing number of calls to hasattr
• Makes clear_cache() work on subpackets

[codecov]

Codecov Report

Merging #2035 into master will increase coverage by 0.01%.
The diff coverage is 92.5%.

@@            Coverage Diff             @@
##           master    #2035      +/-   ##
==========================================
+ Coverage   86.31%   86.32%   +0.01%     
==========================================
  Files         197      197              
  Lines       44468    44492      +24     
==========================================
+ Hits        38383    38409      +26     
+ Misses       6085     6083       -2

Impacted Files	Coverage Δ
scapy/volatile.py	79.47% <100%> (+0.09%)	⬆️
scapy/fields.py	91.72% <100%> (+0.01%)	⬆️
scapy/packet.py	79.48% <90.62%> (+0.32%)	⬆️
scapy/config.py	83.69% <0%> (-0.87%)	⬇️
scapy/layers/ntp.py	91.51% <0%> (-0.54%)	⬇️
scapy/sendrecv.py	85.21% <0%> (-0.17%)	⬇️
scapy/layers/inet6.py	88.1% <0%> (-0.12%)	⬇️
scapy/asn1/ber.py	83.09% <0%> (+0.28%)	⬆️
scapy/layers/tls/basefields.py	80.53% <0%> (+0.67%)	⬆️
... and 3 more

[gpotter2]

Some comments for the reviewers

[gpotter2]

This also made very little sense: it abuses the bugs in __iter__ to generate a list of packets, whereas it should only have been a flag field.

[gpotter2]

For some reason I missed that: (0, 5) makes 6 values... -_-

[gpotter2]

This is actually quite a big improvement: in fact hasattr calls __getattr__ under the hood, which calls getfieldval => we actually did getfieldval twice each time.

[gpotter2]

This was triggered by the FlagsField bug

[guedou]

@p-l- can you review this PR?