It was a wrong assumption that `a <> 1 and a <> 2)` is equal to
`(a, a) <> (1, 2)`. Fixed.

As a side effect some refactoring was made to deal with code
duplication in the cartridge, mock and bench engines.

Previously, only the table cache was invalidated on the router. Now
we flush cached IRs as well.

Thanks to the updates in tarantool module we don't use tarantool
symbols to work with decimal. As a bonus we can remove our mocking
framework with dynamic linking of the decNumber library to cargo
test binary.

As a side effect the code of explain was refactored and simplified.

As a side effect we got rid of the environment variable in a unit
tests build. Now we rely of a feature "mock" for this purpose.

1. When we build a configuration cache and face a table without
   a sharding column, simply skip this table (do not exit with
   error).
2. Make "space not found" error more debug firendly
3. Rename "broken test" to "ddl"

BREAKING CHANGE: reqiures Picodata Tarantool with a string quoting
fix (e9360c989129a755b86486c7bcd6e2d21a078e22).

We should not expose a sharding column (i.e. "bucket_id") for any
parts of the join if the user doesn't mention it explicitly in
projection. Otherwise we receive incorrect query results.

The error was produced by the assumption that (2, 1) > (1, 2) is
equivalent to (2 > 1) and (1 > 2). That assumption was incorrect,
so we don't merge AND chains for any operators other than Eq and
NotEq.

BREAKING CHANGE: dispatch_query() wraps result with a tuple of a
single element.

We made a wrong enumeration for the new columns in projection
when a system sharding column ("bucket_id") was located somewhere
except the last position in the relation schema. Fixed.

We should not panic if we try to clear the configuration while
holding a mutable borrow on a runtime.

Previously we didn't take into account the fact, that prepared
statements act much alike OS descriptors. Though their internal
representation is located in the instance-wide memory of Tarantool,
we need to call "prepare" method to make its id from the global
cache for every new session. If the statement was not found in the
cache, then "prepare" compiles it and stores back to the cache.

Previously we failed to deserialize decimal

BREAKING CHANGE: now we need sbroad library on the storages as well

BREAKING CHANGE: rename execute_query to dispatch_query

The main reason is that we'll soon need not only a coordinator
runtime, but also a new storage runtime as well. Current names
can confuse us in future.

Also add an ability to evaluate an arbitrary eviction funtion for
the LRU cache (on the storage side we'll need it to flush prepared
statements from the Tarantool cache when our LRU cache evicts the
one).

LRU cache was moved from within the cache folder into the engine.

Router logic was moved to a separate file in the cartridge module.