ref(batcher): name dedicated batcher threads

[jpnurmi]

Give the logs and metrics batchers distinct OS-level thread names.

Named dedicated threads make profiling and tracing easier because background work can be identified directly in debuggers, crash dumps, and trace timelines instead of showing up as anonymous worker activity.

Before

$ ps -T -p $(pidof sentry-playground) -o pid,tid,comm
    PID     TID COMMAND
  88207   88207 sentry-playgrou
  88207   88208 QDBusConnection
  88207   88210 [pango] fontcon
  88207   88211 pool-spawner
  88207   88212 gmain
  88207   88213 pool-0
  88207   88214 gdbus
  88207   88215 dconf worker
  88207   88216 WaylandEventThr
  88207   88217 WaylandEventThr
  88207   88225 Thread (pooled)
  [...]
  88207   88232 Thread (pooled)
  88207   88265 sentry-http
  88207   88268 sentry-playgrou     ### <===
  88207   88269 sentry-playgrou     ### <===

After

$ ps -T -p $(pidof sentry-playground) -o pid,tid,comm
    PID     TID COMMAND
  86707   86707 sentry-playgrou
  86707   86708 QDBusConnection
  86707   86710 [pango] fontcon
  86707   86711 pool-spawner
  86707   86712 gmain
  86707   86714 gdbus
  86707   86715 dconf worker
  86707   86716 WaylandEventThr
  86707   86717 WaylandEventThr
  86707   86720 Thread (pooled)
  [...]
  86707   86727 Thread (pooled)
  86707   86732 sentry-http
  86707   86735 sentry-logs         ### <===
  86707   86736 sentry-metrics      ### <===

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.80%. Comparing base (c3bca45) to head (84a0453).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1937      +/-   ##
==========================================
- Coverage   75.80%   75.80%   -0.01%     
==========================================
  Files          93       93              
  Lines       22149    22167      +18     
  Branches     3942     3944       +2     
==========================================
+ Hits        16791    16804      +13     
- Misses       4477     4481       +4     
- Partials      881      882       +1     

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