Closes #1082

@TarantoolBot document
Title: Document box.cfg.checkpoint_wal_threshold

Tarantool makes checkpoints every box.cfg.checkpoint_interval seconds
and keeps last box.cfg.checkpoint_count checkpoints. It also keeps all
intermediate WAL files. Currently, it isn't possible to set a checkpoint
trigger based on the sum size of WAL files, which makes it difficult to
estimate the minimal amount of disk space that needs to be allotted to a
Tarantool instance for storing WALs to eliminate the possibility of
ENOSPC errors. For example, under normal conditions a Tarantool instance
may write 1 GB of WAL files every box.cfg.checkpoint_interval seconds
and so one may assume that 1 GB times box.cfg.checkpoint_count should be
enough for the WAL partition, but there's no guarantee it won't write 10
GB between checkpoints when the load is extreme.

So we've agreed that we must provide users with one more configuration
option that could be used to impose the limit on the sum size of WAL
files. The new option is called box.cfg.checkpoint_wal_threshold. Once
the configured threshold is exceeded, the WAL thread notifies the
checkpoint daemon that it's time to make a new checkpoint and delete
old WAL files. Note, the new option only limits the size of WAL files
created since the last checkpoint, because backup WAL files are not
needed for recovery and can be deleted in case of emergency ENOSPC, for
more details see tarantool/tarantool#1082, tarantool/tarantool#3397,
tarantool/tarantool#3822.

The default value of the new option is 1 exabyte (10^18 byte), which
actually means that the feature is disabled.