By default use the platform convention (i.e. "\r\n" on windows), but
make it possible to set a custom end-of-line string without implementing
a custom formatter.

(Note: this can be helpful when a custom sink invokes a JNI callback, which implies that worker thread was previously attached to JVM, and needs to be cleanly detached upon exit)

update comments

Fixed issue #179 (Conflict with Boost.Asio) (by defining FMT_USE_WINDOWS_H=0 in format.h and preventing include of windows.h)

assigned nullptr to _database and _query_stmt upon flushing so that if flush gets called again, will not throw an exception

changed ctrs,variables,argsguments to match code style of spdlog, also removed columns mapping since there is no clean way of set column name dynamically on a prepared statement and changed query to use prepared statements