Add SQLite support, fix un-thread-safe DB caches, small performance f… (#172)

* Add SQLite support, fix un-thread-safe DB caches, small performance fixes

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* add SQLite licenses to README

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* appease the linter, and fix my dumbass-ery

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* make requested changes

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* add back comment

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

internal/db/bundb/errors.go

@@ -0,0 +1,43 @@
+package bundb
+
+import (
+	"github.com/jackc/pgconn"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"modernc.org/sqlite"
+	sqlite3 "modernc.org/sqlite/lib"
+)
+
+// processPostgresError processes an error, replacing any postgres specific errors with our own error type
+func processPostgresError(err error) db.Error {
+	// Attempt to cast as postgres
+	pgErr, ok := err.(*pgconn.PgError)
+	if !ok {
+		return err
+	}
+
+	// Handle supplied error code:
+	// (https://www.postgresql.org/docs/10/errcodes-appendix.html)
+	switch pgErr.Code {
+	case "23505" /* unique_violation */ :
+		return db.ErrAlreadyExists
+	default:
+		return err
+	}
+}
+
+// processSQLiteError processes an error, replacing any sqlite specific errors with our own error type
+func processSQLiteError(err error) db.Error {
+	// Attempt to cast as sqlite
+	sqliteErr, ok := err.(*sqlite.Error)
+	if !ok {
+		return err
+	}
+
+	// Handle supplied error code:
+	switch sqliteErr.Code() {
+	case sqlite3.SQLITE_CONSTRAINT_UNIQUE:
+		return db.ErrAlreadyExists
+	default:
+		return err
+	}
+}