Accept String or FixedString data types as key for maps in the
SummingMergeTree now that sumMap supports these two types (introduced in
bc0fbd68).
Signed-off-by: NBaudouin Giard <bgiard@bloomberg.net>

Now Distributed() has gain the 5-th argument -- policy name (for storing
data to send):

  CREATE TABLE foo (key Int) Engine=Distributed(test_shard_localhost, currentDatabase(), some_table, key%2, 'default');

* Add virtual column _path to hdfs and file table functions with test

* Fix const of headers

* Add column _file with tests

* Add docs

* Fix improper resolve conflicts

* Fix links in docs

* Better condition for virtual columns proccessing in StorageFile

* better condition for virtual columns processing in StorageHDFS

docs(merge_tree_min_bytes_for_concurrent_read, merge_tree_max_bytes_to_use_cache):The description of `merge_tree_min_bytes_for_concurrent_read, merge_tree_max_bytes_to_use_cache` function was edited.

* Update mergetree.md

added storage_configuration tag into examples

* Update mergetree.md

added storage_configuration tag to examples