Design conditional multi update functionality for meta storage client

Define entities required for implementation of conditional multi updates (aka "transactions") for meta storage.

Requirements:

• Design should provide possibility to define conditional updates in if-then-else manner with nested branches.
• Update always starts with condition.
• Any nested branch must start with condition.
• Update can't precede to condition.
• Every branch can yield exactly one custom result (limited by number of simple types) and any number of entries. Corresponding expression always must be at the end of branch.
• Condition for entry revision allows the following comparisons: equal, !equal, less, greater.
• Condition for entry value allows the following operations (byte-wise): equal, !equal.
• Update allows the following operations: put, remove, no-op.
• Condition can not use previously defined condition instance again because it will lead to graph cycle and infinite update execution.

Assumptions:

• It seems that conditions and updates entities can be conveniently linked using chaining.

Simple example (pseudocode):

    CompletableFuture<Result, List<Entry>> = invoke(valCond(EQUAL, perstEnabled, true)._then(
             valCond(EQUAL, walEnabled, true)._then(
                     update(walEnabled, true)).
             _else(
                     update(walEnabled, false))
            )._else(NoopUpdate))