Fixes gh-623.

An implementation for both unique and non-unique
secondary indexes for vinyl storage engine.
engine.

Tests for secondary indexes were added to 'engine' test suite.

* only run the garbage collector in tx manager if there are
  conditions for it.

- Remove useless abstraction layers
- Extract explicit vy_task structure from code
- Fix #1558: occasional crashes after dropping index

Needed for #1592

* remove a separate counter for commit sequence
  number, put transaction start and end time on the same logical
  clock scale

Package.searchpath, if probed XXX.dylib and failed will attempt
XXX.so before moving to next path element (osx).

Package.searchpath, if probed XXX.dylib and failed will attempt
XXX.so before moving to next path element (osx).

Fixes #810

* don't recycle input buffer if this leads to growth of the output
buffer

Needed for secondary keys in vinyl (#623)

Needed to remove VINYL_LSN sequence (#1609).

The only case when the tx manager was used from the backend
was to query the oldest active read view. Instead of querying
from the backend, maintain a sequence and update it
whenever the oldest read view is advanced.

I failed to come up with any resonable test case when the code
would be triggered/do something useful.

* add insert test ase to mvcc.test
* convert hermitage.test to client connections (vs. fibers)

- Remove a, b, c; use union instread.
- Remove unused srzone->compact_mode

Remove artifacts of anticache mode.

Closes #1595

Remove artifacts of anticache mode.

* add basic transactional tests
* port functional tests from C to Lua

No semantic changes.

tarantool/libyaml was re-created directly from hg repo
using hg-fast-export. Code is not changed.

* remove a completely redundant struct

* remove a completely redundant member

Use (key, part_count) and `struct tuple` instead.
See #1572

* remove overengineering