Change the default Policy of segment_vector
from exponential_size_array_policy to exponential_size_vector_policy.

enumerable_thread_specific has to have 'exponential_size_array_policy'
as Storage, because its storage must not do reallocations.

enumerable_thread_specific: fix

fixed datarace and test added

Add variable to run long tests

Enumerable_thread_specific fix

Add workflow file for GitHub actions

Segment vector fixes

If the 'TESTS_LONG' environment variable is set to ON,
the long tests will be run (the default value is OFF).

String find

Comment added with explanation to "upgrade_to_writer" in "local" method

Segment_vector fix

Separate implementation for find(CharT ch, size_type pos) overload
is redundant and may be changed to just calling basic implementation
with specified parameters

Separate implementation for rfind(CharT ch, size_type pos) overload
is redundant and may be changed to just calling basic implementation
with specified parameters

Recursion on a long strings may cause stack overflow.
Previous implementation was tricky, as at first glance
rfind( CharT ch, size_type pos = npos ) overload is called and
everything is ok, but it calls
rfind(const CharT *s, size_type pos, size_type count) under the hood.

For consistency use std::distance for pointer arithmetic in every
overload of find

Skip redundant while loop cycles by using traits_type::find return
value, instead of just checking if it finds anything over and over.

Added additional checks in methods to avoid possible issues in case of numeric arguments with value equal to zero.
Implemented test to check these methods.

Merge changes from stable 1.8

Change owner and group of HOST_WORKDIR

Simplify travis.yml

The 'HOST_WORKDIR' directory contains the source code
cloned in a CI, so UID and GID of its files are equal
to UID and GID of a CI user, which can vary depending on a CI.
We have to change the ownership of this directory
and all its files to UID and GID of the Docker user,
because the build is run in Docker.