compiler warnings on implicit capture of "this" in client.cpp

when compiler as a submodule with g++ v11, i get many of these compiler warnings;
```
sources/core/client.cpp:...:19: note: add explicit ‘this’ or ‘*this’ capture
sources/core/client.cpp: In lambda function:
sources/core/client.cpp:...:19: warning: implicit capture of ‘this’ via ‘[=]’ is deprecated in C++20 [-Wdeprecated]           
```