Automatic compaction selection on Linux (#2785)

* add auto compaction types

* pass db paths

* detect drive type on Linux

* use base db path

* add docstring

* limit the test to be side effect free

* use base db path

* more docs

* fix parsing test

* update error

* detect only on Linux

* make test Linux only

* add second device letter, update cli doc

* use spaces in cli doc

* import only on linux

* default->auto

parity/cli/mod.rs

@@ -225,7 +225,7 @@ usage! {
 			or |c: &Config| otry!(c.footprint).cache_size.clone().map(Some),
 		flag_fast_and_loose: bool = false,
 			or |c: &Config| otry!(c.footprint).fast_and_loose.clone(),
-		flag_db_compaction: String = "ssd",
+		flag_db_compaction: String = "auto",
 			or |c: &Config| otry!(c.footprint).db_compaction.clone(),
 		flag_fat_db: String = "auto",
 			or |c: &Config| otry!(c.footprint).fat_db.clone(),

parity/cli/usage.txt

@@ -232,7 +232,8 @@ Footprint Options:
                            but means an unclean exit is unrecoverable. (default: {flag_fast_and_loose})
   --db-compaction TYPE     Database compaction type. TYPE may be one of:
                            ssd - suitable for SSDs and fast HDDs;
-                           hdd - suitable for slow HDDs (default: {flag_db_compaction}).
+                           hdd - suitable for slow HDDs;
+                           auto - determine automatically (default: {flag_db_compaction}).
   --fat-db BOOL            Build appropriate information to allow enumeration
                            of all accounts and storage keys. Doubles the size
                            of the state database. BOOL may be one of on, off