Use a fixed regex for StripState

Use a fixed regex for StripState

The JIT compiler in newer versions of PCRE experiences lock contention
when multithreaded applications perform a high rate of concurrent
compilations. We are seeing some performance impact on HHVM under normal
production traffic.

The random part of the strip marker is just there to protect against
deliberate insertion of strip markers into the source text, which is
very rare. So use a generic regex to find strip markers, and check in
the callback whether the random state ID is correct.

StripState::killMarkers() will be slower when it has to remove many
strip markers, but most calls to it will not match any strip markers, so
overall performance should be improved due to reduced JIT compilation.

Bug: 72205
Change-Id: I8d37ae929a8c669c9e39adc8096b89e5732b68d0