This commit extends compatibility with the bbolt kvdb implementation,
which returns ErrIncompatibleValue in case of a bucket/value key
collision. Furthermore the commit also adds an extra precondition to the
transaction when a key doesn't exist. This is needed as we fix reads to
a snapshot revision and other writers may commit the key otherwise.
This commit changes the key derivation algo we use to emulate buckets
similar to bbolt. The issue with prefixing keys with either a bucket or
a value prefix is that the cursor couldn't effectively iterate trough
all keys in a bucket, as it skipped the bucket keys.
While there are multiple ways to fix that issue (eg. two pointers,
iterating value keys then bucket keys, etc), the cleanest is to instead
of prefixes in keys we use a postfix indicating whether a key is a
bucket or a value. This also simplifies all operations where we
(recursively) iterate a bucket and is equivalent with the prefixing key
derivation with the addition that bucket and value keys are now
continous.
This commit separates all etcd related sources (sans a few stubs and
config) from the rest of the source tree and makes compilation conditional
depending on whether the kvdb_etcd build tag is specified.
