Always start performance tracing

[stayallive]

As mentioned in #595, we are incorrectly "guarding" against enabling performance tracing when no sample rate or sampler callback is set. This removes that check and always enables performance tracing.

This has minimal impact on the users application since we always check if there is an active transaction/span before doing any real work so most code becomes a no-op if the transaction is not sampled.

This should be merged to master after #587.

This will also have no real work effect until getsentry/sentry-php#1405 has been solved.

[cleptric]

h: We have to bump sentry/sentry to ^3.10.
And while we're at it, sentry/sdk to ^3.3.

h: Middleware::hydrateResponseData() throws an Call to a member function setHttpStatus() on null error, when traces_sample_rate is set to0.