The backup worker needs to use this version for popping when running in a NOOP
mode. This option is added to GetReadVersionRequest and proxies will send back
minKnownCommittedVersion if the option is set.
Also add a couple of knobs for backup workers.
The backup worker just blindly pop tags if the "backupStartedKey" is not set.
Note the commit version from TLog cannot be used as the pop version, because
for a single region, during a recovery the log router tags are used to recover
mutations. The backup worker can potentially pop mutations that are needed for
recovery, causing consistency errors. So the solution for now is to use commit
version - 5,000,000, which is a version guaranteed to be persisted on all
replicas.
For each mutation, its version, sub-version, and size are prefixed with big
endian representation. This is required, especially for the first version
variable, because we use 0xFF for padding purpose. A little endian version
number can easily collide with 0xFF, while big endian is guaranteed to have
0x00 as the first byte.
Evan Tschannen