Set thread mutex to the DNSHandler mutex of SplitDNS (#7321)

[masaori335]

Backport #7321 against the 8.1.x branch.

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(cherry picked from commit 3f11f15)

[masaori335]

[approve ci freebsd]
[approve ci fedora]

[bryancall]

@zwoop are you going to cherry pick this in?

[zwoop]

Yeh, I think we should pick this one.

[masaori335]

For CIs, we can't build 8.1.x on Fedora anyway because of TCL dependency. The failure on the FreeBSD seems unrelated to this PR, it's wired though.

libtool: compile:  ccache clang -DHAVE_CONFIG_H -I. -I../../include -D_GLIBCXX_USE_C99 -D_GLIBCXX_USE_C99_MATH -D_GLIBCXX_USE_C99_MATH_TR1 -Dfreebsd -D_LARGEFILE64_SOURCE=1 -D_COMPILE64BIT_SOURCE=1 -D_REENTRANT -D__STDC_LIMIT_MACROS=1 -D__STDC_FORMAT_MACROS=1 -I/var/jenkins/workspace/freebsd-github/src/include -I/var/jenkins/workspace/freebsd-github/src/lib -I/usr/local/include -I/usr/local/include/tcl8.6 -D_GNU_SOURCE -I/usr/include -DOPENSSL_NO_SSL_INTERN -I/usr/local/include -std=gnu99 -g -pipe -Wall -Wno-deprecated-declarations -Qunused-arguments -Wextra -Wno-ignored-qualifiers -Wno-unused-parameter -O3 -fno-strict-aliasing -Werror -mcx16 -MT TsConfigSyntax.lo -MD -MP -MF .deps/TsConfigSyntax.Tpo -c TsConfigSyntax.c  -fPIC -DPIC -o .libs/TsConfigSyntax.o
In file included from TsBuilder.cc:27:
./TsConfigGrammar.hpp:21:16: error: ISO C++ forbids forward references to 'enum' types
  typedef enum yytokentype yytoken_kind_t;
               ^
1 error generated.