Allocate buffer memory for uv_write and release in the callback function

[stelfrag]

Fixes #12685

Summary

Allocate buffers needed for uv_write and let the callback function release the memory

Test Plan

• Alert notifications should work

[thiagoftsm]

I tested the PR, and I received all expected notifications. I also used valgrind 3.17, and I did not receive memory leak notification for this code. LGTM!

[thiagoftsm]

@stelfrag the next warning does not have relationship with your PR, but I observed it during my tests:

==8655== 848 bytes in 1 blocks are still reachable in loss record 2,659 of 2,764
==8655==    at 0x48467C5: malloc (vg_replace_malloc.c:380)
==8655==    by 0x431C79: mallocz (libnetdata.c:176)
==8655==    by 0x574D8B: spawn_client (spawn_client.c:176)
==8655==    by 0x5627052: start_thread (in /lib64/libc-2.35.so)
==8655==    by 0x56B3BEF: clone (in /lib64/libc-2.35.so)

I also observed that we are not cleaning properly the spawn server:

==8655== 416 bytes in 1 blocks are possibly lost in loss record 2,616 of 2,764
==8655==    at 0x484B5FF: calloc (vg_replace_malloc.c:1117)
==8655==    by 0x4013F95: _dl_allocate_tls (in /lib64/ld-2.35.so)
==8655==    by 0x5627D24: pthread_create@@GLIBC_2.34 (in /lib64/libc-2.35.so)
==8655==    by 0x4E1EECC: uv_thread_create_ex (in /usr/lib64/libuv.so.1.0.0)
==8655==    by 0x4E1EF7B: uv_thread_create (in /usr/lib64/libuv.so.1.0.0)
==8655==    by 0x5738C6: spawn_init (spawn.c:243)
==8655==    by 0x41BF8C: main (main.c:1218)

I got the messages running:

valgrind --leak-check=full --show-leak-kinds=all --log-file=errors.log  /usr/sbin/netdata -D

best regards!