do not use ScriptVersionCache for closed files

[vladima]

ScriptVersionCache internally stores text in tree-like form which is very efficient for editing purposes and at the same time pretty expensive to be used as a generic storage. Size of the tree is proportional to the size of the file and varies from 1.5 x (size of text) for small files through to 3x up to 10x in a degenerate cases (like 40 MB files). As an example android.d.ts is ~13 MB file and its tree representation is takes roughly 40 MB.

To avoid paying this cost I've introduced TextStorage abstraction that internally can either store text as is or use ScriptVersionCache. ScriptVersionCache is used for open files or for some operations that cannot be efficiently used on plain text (like editing). For all other cases it uses plain text deferring its loading (which is useful for scenarios when language service is disabled).

// cc @mhegazy

[mhegazy]

we need to port these into release-2.1 as well.