Use workspace locks in requests

Example steps to show problem:

Load a very large workspace (i.e., Hugo)
Before the project has finished loading, try to get the definition of an ident

Error log looks something like:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x14aba3c]

goroutine 1872 [running]:
github.com/object88/langd.(*Workspace).LocateDeclaration.func1(0x1904ba0, 0xc42092f920, 0x12698)
    /Users/bropa18/work/src/github.com/object88/langd/workspace.go:183 +0x36c
go/ast.inspector.Visit(0xc43df39e20, 0x1904ba0, 0xc42092f920, 0x23, 0xc420db9770)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:373 +0x3a
go/ast.Walk(0x1901be0, 0xc43df39e20, 0x1904ba0, 0xc42092f920)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:52 +0x66
go/ast.Walk(0x1901be0, 0xc43df39e20, 0x19042e0, 0xc4209e1480)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:136 +0x1041
go/ast.walkExprList(0x1901be0, 0xc43df39e20, 0xc420a6fcf0, 0x1, 0x1)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:26 +0x81
go/ast.Walk(0x1901be0, 0xc43df39e20, 0x19040e0, 0xc4209e14c0)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:207 +0x2122
go/ast.walkStmtList(0x1901be0, 0xc43df39e20, 0xc420eb5300, 0x8, 0x8)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:32 +0x81
go/ast.Walk(0x1901be0, 0xc43df39e20, 0x1904260, 0xc420975530)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:224 +0x1b7a
go/ast.Walk(0x1901be0, 0xc43df39e20, 0x19046e0, 0xc420975560)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:344 +0xd83
go/ast.walkDeclList(0x1901be0, 0xc43df39e20, 0xc420cf0a00, 0x9, 0x10)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:38 +0x81
go/ast.Walk(0x1901be0, 0xc43df39e20, 0x1904660, 0xc420eb5380)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:353 +0x2672
go/ast.Inspect(0x1904660, 0xc420eb5380, 0xc43df39e20)
    /usr/local/Cellar/go/1.9.3/libexec/src/go/ast/walk.go:385 +0x4b
github.com/object88/langd.(*Workspace).LocateDeclaration(0xc420170450, 0xc440c8ea50, 0xc42092f900, 0x0, 0x0)
    /Users/bropa18/work/src/github.com/object88/langd/workspace.go:160 +0x277
github.com/object88/langd/requests.(*definitionHandler).work(0xc42a85ed40, 0x756ea1054a2a01, 0xc43aad67c0)
    /Users/bropa18/work/src/github.com/object88/langd/requests/definition.go:72 +0x7c
github.com/object88/langd/requests.(*Handler).finishProcessing(0xc42008f860, 0x190d140, 0xc42a85ed40)
    /Users/bropa18/work/src/github.com/object88/langd/requests/handler.go:197 +0x35
created by github.com/object88/langd/requests.(*Handler).startProcessing
    /Users/bropa18/work/src/github.com/object88/langd/requests/handler.go:193 +0x109

Problem is suspected to be that once the project is opened, we handle the didOpen request, which will cause the package to reload. We immediately then run definition, but the reload hasn't finished, so we get a nil package pointer. This is bad, m'key?

object88 / langd

Use workspace locks in requests #29