This adds handling for various corruption scenarios of the WAL.
If corruption is encountered, we truncate the WAL after the last valid
entry transparently and continue appending after the offset.
This fixes a bug where the last WAL file was closed after consuming it
instead of being left open for further writes.
Reloading of blocks on startup considers loading head blocks now.
Introduce a seperate mutex for the head blocks to avoid a race where
a post-compaction reload may run between switching the DB's base mutex
to create a new head block in an appender.
File locks have a multitude of problems that make them hard to use
correctly. As they are just advisory, they are only meaningful to
prevent accidents like running the same process twice.
A simple PID file lock works reliably in those cases and is simpler.
The former approach created unordered postings list by either
map iteration of new series being unsorted (fixable) or concurrent
writers creating new series interleaved.
We switch back to generating ephemeral references for a single batch.
Newly created series have to be re-set upon the next insert.
Fabian Reinartz