Readers serving user-reads need to obtain a permit to start reading.
There exists a restriction on how much active readers can be admitted
based on their count and their memory onsumption.
Since the saved readers of cached queriers are techically active (they
hold a permit) they can block new readers from obtaining a permit.
New readers have a higher priority because a cached reader might be
abandoned or used later at best so in the face of memory pressure we
evict cached readers to free up permits for new readers.
Cached queriers are evicted in LRU order as the oldest queriers are the
most likely to be evicted based on their TTL anyway.
This semaphore implements the new dual, count and memory based active
reader limiting. As purely memory-based limiting proved to cause
problems on big boxes admitting a large number of readers (more than any
disk could handle) the previous count-based limit is reintroduced in
addition to the existing memory-based limit.
When creating new readers first the count-based limit is checked. If
that clears the memory limit is checked before admitting the reader.
reader_conccurency_semaphore wraps the two semaphores that implement
these limits and enforces the correct order of limit checking.
This class also completely replaces the restricted_reader_config struct,
it encapsulates all data and related functinality of the latter, making
client code simpler.
In preparation to reader_concurrency_semaphore being added to the file.
The reader_resource_tracker is really only a helper class for
reader_concurrency_semaphore so the latter is better suited to provide
the name of the file.
